From f2bcd05ec7b839ff826d2008506ad2d2dff46a59 Mon Sep 17 00:00:00 2001 From: Andrey Ignatov Date: Wed, 3 Apr 2019 23:22:37 -0700 Subject: [PATCH 1/7] bpf: Reject indirect var_off stack access in raw mode It's hard to guarantee that whole memory is marked as initialized on helper return if uninitialized stack is accessed with variable offset since specific bounds are unknown to verifier. This may cause uninitialized stack leaking. Reject such an access in check_stack_boundary to prevent possible leaking. There are no known use-cases for indirect uninitialized stack access with variable offset so it shouldn't break anything. Fixes: 2011fccfb61b ("bpf: Support variable offset stack access from helpers") Reported-by: Daniel Borkmann Signed-off-by: Andrey Ignatov Signed-off-by: Daniel Borkmann --- kernel/bpf/verifier.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index bb27b675923c..0f12fda35626 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2226,6 +2226,15 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno, if (err) return err; } else { + /* Only initialized buffer on stack is allowed to be accessed + * with variable offset. With uninitialized buffer it's hard to + * guarantee that whole memory is marked as initialized on + * helper return since specific bounds are unknown what may + * cause uninitialized stack leaking. + */ + if (meta && meta->raw_mode) + meta = NULL; + min_off = reg->smin_value + reg->off; max_off = reg->umax_value + reg->off; err = __check_stack_boundary(env, regno, min_off, access_size, From f68a5b44647bce6c34b10d5560c5b2c0aff31afc Mon Sep 17 00:00:00 2001 From: Andrey Ignatov Date: Wed, 3 Apr 2019 23:22:38 -0700 Subject: [PATCH 2/7] selftests/bpf: Test indirect var_off stack access in raw mode Test that verifier rejects indirect access to uninitialized stack with variable offset. Example of output: # ./test_verifier ... #859/p indirect variable-offset stack access, uninitialized OK Signed-off-by: Andrey Ignatov Signed-off-by: Daniel Borkmann --- .../testing/selftests/bpf/verifier/var_off.c | 27 +++++++++++++++++++ 1 file changed, 27 insertions(+) diff --git a/tools/testing/selftests/bpf/verifier/var_off.c b/tools/testing/selftests/bpf/verifier/var_off.c index c4ebd0bb0781..3840bd16e173 100644 --- a/tools/testing/selftests/bpf/verifier/var_off.c +++ b/tools/testing/selftests/bpf/verifier/var_off.c @@ -114,6 +114,33 @@ .result = REJECT, .prog_type = BPF_PROG_TYPE_LWT_IN, }, +{ + "indirect variable-offset stack access, uninitialized", + .insns = { + BPF_MOV64_IMM(BPF_REG_2, 6), + BPF_MOV64_IMM(BPF_REG_3, 28), + /* Fill the top 16 bytes of the stack. */ + BPF_ST_MEM(BPF_W, BPF_REG_10, -16, 0), + BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), + /* Get an unknown value. */ + BPF_LDX_MEM(BPF_W, BPF_REG_4, BPF_REG_1, 0), + /* Make it small and 4-byte aligned. */ + BPF_ALU64_IMM(BPF_AND, BPF_REG_4, 4), + BPF_ALU64_IMM(BPF_SUB, BPF_REG_4, 16), + /* Add it to fp. We now have either fp-12 or fp-16, we don't know + * which, but either way it points to initialized stack. + */ + BPF_ALU64_REG(BPF_ADD, BPF_REG_4, BPF_REG_10), + BPF_MOV64_IMM(BPF_REG_5, 8), + /* Dereference it indirectly. */ + BPF_EMIT_CALL(BPF_FUNC_getsockopt), + BPF_MOV64_IMM(BPF_REG_0, 0), + BPF_EXIT_INSN(), + }, + .errstr = "invalid indirect read from stack var_off", + .result = REJECT, + .prog_type = BPF_PROG_TYPE_SOCK_OPS, +}, { "indirect variable-offset stack access, ok", .insns = { From 088ec26d9c2da9d879ab73e3f4117f9df6c566ee Mon Sep 17 00:00:00 2001 From: Andrey Ignatov Date: Wed, 3 Apr 2019 23:22:39 -0700 Subject: [PATCH 3/7] bpf: Reject indirect var_off stack access in unpriv mode Proper support of indirect stack access with variable offset in unprivileged mode (!root) requires corresponding support in Spectre masking for stack ALU in retrieve_ptr_limit(). There are no use-case for variable offset in unprivileged mode though so make verifier reject such accesses for simplicity. Pointer arithmetics is one (and only?) way to cause variable offset and it's already rejected in unpriv mode so that verifier won't even get to helper function whose argument contains variable offset, e.g.: 0: (7a) *(u64 *)(r10 -16) = 0 1: (7a) *(u64 *)(r10 -8) = 0 2: (61) r2 = *(u32 *)(r1 +0) 3: (57) r2 &= 4 4: (17) r2 -= 16 5: (0f) r2 += r10 variable stack access var_off=(0xfffffffffffffff0; 0x4) off=-16 size=1R2 stack pointer arithmetic goes out of range, prohibited for !root Still it looks like a good idea to reject variable offset indirect stack access for unprivileged mode in check_stack_boundary() explicitly. Fixes: 2011fccfb61b ("bpf: Support variable offset stack access from helpers") Reported-by: Daniel Borkmann Signed-off-by: Andrey Ignatov Signed-off-by: Daniel Borkmann --- kernel/bpf/verifier.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 0f12fda35626..8400c1f33cd4 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2226,6 +2226,19 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno, if (err) return err; } else { + /* Variable offset is prohibited for unprivileged mode for + * simplicity since it requires corresponding support in + * Spectre masking for stack ALU. + * See also retrieve_ptr_limit(). + */ + if (!env->allow_ptr_leaks) { + char tn_buf[48]; + + tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off); + verbose(env, "R%d indirect variable offset stack access prohibited for !root, var_off=%s\n", + regno, tn_buf); + return -EACCES; + } /* Only initialized buffer on stack is allowed to be accessed * with variable offset. With uninitialized buffer it's hard to * guarantee that whole memory is marked as initialized on @@ -3339,6 +3352,9 @@ static int retrieve_ptr_limit(const struct bpf_reg_state *ptr_reg, switch (ptr_reg->type) { case PTR_TO_STACK: + /* Indirect variable offset stack access is prohibited in + * unprivileged mode so it's not handled here. + */ off = ptr_reg->off + ptr_reg->var_off.value; if (mask_to_left) *ptr_limit = MAX_BPF_STACK + off; From 2c6927dbdc3fbd41207e671212f53a98bbebf6ba Mon Sep 17 00:00:00 2001 From: Andrey Ignatov Date: Wed, 3 Apr 2019 23:22:40 -0700 Subject: [PATCH 4/7] selftests/bpf: Test indirect var_off stack access in unpriv mode Test that verifier rejects indirect stack access with variable offset in unprivileged mode and accepts same code in privileged mode. Since pointer arithmetics is prohibited in unprivileged mode verifier should reject the program even before it gets to helper call that uses variable offset, at the time when that variable offset is trying to be constructed. Example of output: # ./test_verifier ... #859/u indirect variable-offset stack access, priv vs unpriv OK #859/p indirect variable-offset stack access, priv vs unpriv OK Signed-off-by: Andrey Ignatov Signed-off-by: Daniel Borkmann --- .../testing/selftests/bpf/verifier/var_off.c | 27 +++++++++++++++++++ 1 file changed, 27 insertions(+) diff --git a/tools/testing/selftests/bpf/verifier/var_off.c b/tools/testing/selftests/bpf/verifier/var_off.c index 3840bd16e173..f5d5ff18ef22 100644 --- a/tools/testing/selftests/bpf/verifier/var_off.c +++ b/tools/testing/selftests/bpf/verifier/var_off.c @@ -114,6 +114,33 @@ .result = REJECT, .prog_type = BPF_PROG_TYPE_LWT_IN, }, +{ + "indirect variable-offset stack access, priv vs unpriv", + .insns = { + /* Fill the top 16 bytes of the stack. */ + BPF_ST_MEM(BPF_DW, BPF_REG_10, -16, 0), + BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), + /* Get an unknown value. */ + BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 0), + /* Make it small and 4-byte aligned. */ + BPF_ALU64_IMM(BPF_AND, BPF_REG_2, 4), + BPF_ALU64_IMM(BPF_SUB, BPF_REG_2, 16), + /* Add it to fp. We now have either fp-12 or fp-16, we don't know + * which, but either way it points to initialized stack. + */ + BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_10), + /* Dereference it indirectly. */ + BPF_LD_MAP_FD(BPF_REG_1, 0), + BPF_EMIT_CALL(BPF_FUNC_map_lookup_elem), + BPF_MOV64_IMM(BPF_REG_0, 0), + BPF_EXIT_INSN(), + }, + .fixup_map_hash_8b = { 6 }, + .errstr_unpriv = "R2 stack pointer arithmetic goes out of range, prohibited for !root", + .result_unpriv = REJECT, + .result = ACCEPT, + .prog_type = BPF_PROG_TYPE_CGROUP_SKB, +}, { "indirect variable-offset stack access, uninitialized", .insns = { From 107c26a70ca81bfc33657366ad69d02fdc9efc9d Mon Sep 17 00:00:00 2001 From: Andrey Ignatov Date: Wed, 3 Apr 2019 23:22:41 -0700 Subject: [PATCH 5/7] bpf: Sanity check max value for var_off stack access As discussed in [1] max value of variable offset has to be checked for overflow on stack access otherwise verifier would accept code like this: 0: (b7) r2 = 6 1: (b7) r3 = 28 2: (7a) *(u64 *)(r10 -16) = 0 3: (7a) *(u64 *)(r10 -8) = 0 4: (79) r4 = *(u64 *)(r1 +168) 5: (c5) if r4 s< 0x0 goto pc+4 R1=ctx(id=0,off=0,imm=0) R2=inv6 R3=inv28 R4=inv(id=0,umax_value=9223372036854775807,var_off=(0x0; 0x7fffffffffffffff)) R10=fp0,call_-1 fp-8=mmmmmmmm fp-16=mmmmmmmm 6: (17) r4 -= 16 7: (0f) r4 += r10 8: (b7) r5 = 8 9: (85) call bpf_getsockopt#57 10: (b7) r0 = 0 11: (95) exit , where R4 obviosly has unbounded max value. Fix it by checking that reg->smax_value is inside (-BPF_MAX_VAR_OFF; BPF_MAX_VAR_OFF) range. reg->smax_value is used instead of reg->umax_value because stack pointers are calculated using negative offset from fp. This is opposite to e.g. map access where offset must be non-negative and where umax_value is used. Also dedicated verbose logs are added for both min and max bound check failures to have diagnostics consistent with variable offset handling in check_map_access(). [1] https://marc.info/?l=linux-netdev&m=155433357510597&w=2 Fixes: 2011fccfb61b ("bpf: Support variable offset stack access from helpers") Reported-by: Daniel Borkmann Signed-off-by: Andrey Ignatov Signed-off-by: Daniel Borkmann --- kernel/bpf/verifier.c | 18 +++++++++++++++--- 1 file changed, 15 insertions(+), 3 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 8400c1f33cd4..f2d600199e66 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -2248,16 +2248,28 @@ static int check_stack_boundary(struct bpf_verifier_env *env, int regno, if (meta && meta->raw_mode) meta = NULL; + if (reg->smax_value >= BPF_MAX_VAR_OFF || + reg->smax_value <= -BPF_MAX_VAR_OFF) { + verbose(env, "R%d unbounded indirect variable offset stack access\n", + regno); + return -EACCES; + } min_off = reg->smin_value + reg->off; - max_off = reg->umax_value + reg->off; + max_off = reg->smax_value + reg->off; err = __check_stack_boundary(env, regno, min_off, access_size, zero_size_allowed); - if (err) + if (err) { + verbose(env, "R%d min value is outside of stack bound\n", + regno); return err; + } err = __check_stack_boundary(env, regno, max_off, access_size, zero_size_allowed); - if (err) + if (err) { + verbose(env, "R%d max value is outside of stack bound\n", + regno); return err; + } } if (meta && meta->raw_mode) { From 07f9196241f8bceef975dd15f894f8ed51425d55 Mon Sep 17 00:00:00 2001 From: Andrey Ignatov Date: Wed, 3 Apr 2019 23:22:42 -0700 Subject: [PATCH 6/7] selftests/bpf: Test unbounded var_off stack access Test the case when reg->smax_value is too small/big and can overflow, and separately min and max values outside of stack bounds. Example of output: # ./test_verifier #856/p indirect variable-offset stack access, unbounded OK #857/p indirect variable-offset stack access, max out of bound OK #858/p indirect variable-offset stack access, min out of bound OK Signed-off-by: Andrey Ignatov Signed-off-by: Daniel Borkmann --- .../testing/selftests/bpf/verifier/var_off.c | 57 ++++++++++++++++++- 1 file changed, 55 insertions(+), 2 deletions(-) diff --git a/tools/testing/selftests/bpf/verifier/var_off.c b/tools/testing/selftests/bpf/verifier/var_off.c index f5d5ff18ef22..8504ac937809 100644 --- a/tools/testing/selftests/bpf/verifier/var_off.c +++ b/tools/testing/selftests/bpf/verifier/var_off.c @@ -40,7 +40,35 @@ .prog_type = BPF_PROG_TYPE_LWT_IN, }, { - "indirect variable-offset stack access, out of bound", + "indirect variable-offset stack access, unbounded", + .insns = { + BPF_MOV64_IMM(BPF_REG_2, 6), + BPF_MOV64_IMM(BPF_REG_3, 28), + /* Fill the top 16 bytes of the stack. */ + BPF_ST_MEM(BPF_DW, BPF_REG_10, -16, 0), + BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), + /* Get an unknown value. */ + BPF_LDX_MEM(BPF_DW, BPF_REG_4, BPF_REG_1, offsetof(struct bpf_sock_ops, + bytes_received)), + /* Check the lower bound but don't check the upper one. */ + BPF_JMP_IMM(BPF_JSLT, BPF_REG_4, 0, 4), + /* Point the lower bound to initialized stack. Offset is now in range + * from fp-16 to fp+0x7fffffffffffffef, i.e. max value is unbounded. + */ + BPF_ALU64_IMM(BPF_SUB, BPF_REG_4, 16), + BPF_ALU64_REG(BPF_ADD, BPF_REG_4, BPF_REG_10), + BPF_MOV64_IMM(BPF_REG_5, 8), + /* Dereference it indirectly. */ + BPF_EMIT_CALL(BPF_FUNC_getsockopt), + BPF_MOV64_IMM(BPF_REG_0, 0), + BPF_EXIT_INSN(), + }, + .errstr = "R4 unbounded indirect variable offset stack access", + .result = REJECT, + .prog_type = BPF_PROG_TYPE_SOCK_OPS, +}, +{ + "indirect variable-offset stack access, max out of bound", .insns = { /* Fill the top 8 bytes of the stack */ BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), @@ -60,7 +88,32 @@ BPF_EXIT_INSN(), }, .fixup_map_hash_8b = { 5 }, - .errstr = "invalid stack type R2 var_off", + .errstr = "R2 max value is outside of stack bound", + .result = REJECT, + .prog_type = BPF_PROG_TYPE_LWT_IN, +}, +{ + "indirect variable-offset stack access, min out of bound", + .insns = { + /* Fill the top 8 bytes of the stack */ + BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0), + /* Get an unknown value */ + BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 0), + /* Make it small and 4-byte aligned */ + BPF_ALU64_IMM(BPF_AND, BPF_REG_2, 4), + BPF_ALU64_IMM(BPF_SUB, BPF_REG_2, 516), + /* add it to fp. We now have either fp-516 or fp-512, but + * we don't know which + */ + BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_10), + /* dereference it indirectly */ + BPF_LD_MAP_FD(BPF_REG_1, 0), + BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), + BPF_MOV64_IMM(BPF_REG_0, 0), + BPF_EXIT_INSN(), + }, + .fixup_map_hash_8b = { 5 }, + .errstr = "R2 min value is outside of stack bound", .result = REJECT, .prog_type = BPF_PROG_TYPE_LWT_IN, }, From 1fbd20f8b77b366ea4aeb92ade72daa7f36a7e3b Mon Sep 17 00:00:00 2001 From: Andrey Ignatov Date: Wed, 3 Apr 2019 23:22:43 -0700 Subject: [PATCH 7/7] bpf: Add missed newline in verifier verbose log check_stack_access() that prints verbose log is used in adjust_ptr_min_max_vals() that prints its own verbose log and now they stick together, e.g.: variable stack access var_off=(0xfffffffffffffff0; 0x4) off=-16 size=1R2 stack pointer arithmetic goes out of range, prohibited for !root Add missing newline so that log is more readable: variable stack access var_off=(0xfffffffffffffff0; 0x4) off=-16 size=1 R2 stack pointer arithmetic goes out of range, prohibited for !root Fixes: f1174f77b50c ("bpf/verifier: rework value tracking") Signed-off-by: Andrey Ignatov Signed-off-by: Daniel Borkmann --- kernel/bpf/verifier.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index f2d600199e66..48718e1da16d 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -1426,7 +1426,7 @@ static int check_stack_access(struct bpf_verifier_env *env, char tn_buf[48]; tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off); - verbose(env, "variable stack access var_off=%s off=%d size=%d", + verbose(env, "variable stack access var_off=%s off=%d size=%d\n", tn_buf, off, size); return -EACCES; }