mainlining shenanigans
fbb1d4b381
I'm doing some thread necromancy of https://lore.kernel.org/lkml/202007081624.82FA0CC1EA@keescook/ x86, arm64, and arm32 adjusted their READ_IMPLIES_EXEC logic to better align with the safer defaults and the interactions with other mappings, which I illustrated with this comment on x86: /* * An executable for which elf_read_implies_exec() returns TRUE will * have the READ_IMPLIES_EXEC personality flag set automatically. * * The decision process for determining the results are: * * CPU: | lacks NX* | has NX, ia32 | has NX, x86_64 | * ELF: | | | | * ---------------------|------------|------------------|----------------| * missing PT_GNU_STACK | exec-all | exec-all | exec-none | * PT_GNU_STACK == RWX | exec-stack | exec-stack | exec-stack | * PT_GNU_STACK == RW | exec-none | exec-none | exec-none | * * exec-all : all PROT_READ user mappings are executable, except when * backed by files on a noexec-filesystem. * exec-none : only PROT_EXEC user mappings are executable. * exec-stack: only the stack and PROT_EXEC user mappings are * executable. * * *this column has no architectural effect: NX markings are ignored by * hardware, but may have behavioral effects when "wants X" collides with * "cannot be X" constraints in memory permission flags, as in * https://lkml.kernel.org/r/20190418055759.GA3155@mellanox.com * */ For MIPS, the "lacks NX" above is the "!cpu_has_rixi" check. On x86, we decided that the READ_IMPLIES_EXEC flag needed to reflect the expectations, not the architectural behavior due to bad interactions as noted above, as always returning "1" on non-NX hardware breaks some mappings. The other part of the issue is "what does the MIPS toolchain do for PT_GNU_STACK?" The answer seems to be "by default, include PT_GNU_STACK, but mark it executable" (likely due to concerns over non-NX hardware): $ mipsel-linux-gnu-gcc -o hello_world hello_world.c $ llvm-readelf -lW hellow_world | grep GNU_STACK GNU_STACK 0x000000 0x00000000 0x00000000 0x00000 0x00000 RWE 0x10 Given that older hardware doesn't support non-executable memory, it seems safe to make the "PT_GNU_STACK is absent" logic mean "assume non-executable", but this might break very old software running on modern MIPS. This situation matches the ia32-on-x86_64 logic x86 uses (which assumes needing READ_IMPLIES_EXEC in that situation). But modern toolchains on modern MIPS hardware should follow a safer default (assume NX stack). A follow-up to this change would be to switch the MIPS toolchain to emit a non-executable PT_GNU_STACK, as this seems to be unneeded. Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de> Cc: Tiezhu Yang <yangtiezhu@loongson.cn> Cc: Xuefeng Li <lixuefeng@loongson.cn> Cc: Juxin Gao <gaojuxin@loongson.cn> Cc: linux-mips@vger.kernel.org Signed-off-by: Kees Cook <keescook@chromium.org> Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de> |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.