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fd0f86b664
When the TIF_SINGLESTEP tracee dequeues a signal,
handle_signal() clears TIF_FORCED_TF and X86_EFLAGS_TF but
leaves TIF_SINGLESTEP set.
If the tracer does PTRACE_SINGLESTEP again, enable_single_step()
sets X86_EFLAGS_TF but not TIF_FORCED_TF. This means that the
subsequent PTRACE_CONT doesn't not clear X86_EFLAGS_TF, and the
tracee gets the wrong SIGTRAP.
Test-case (needs -O2 to avoid prologue insns in signal handler):
#include <unistd.h>
#include <stdio.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <sys/user.h>
#include <assert.h>
#include <stddef.h>
void handler(int n)
{
asm("nop");
}
int child(void)
{
assert(ptrace(PTRACE_TRACEME, 0,0,0) == 0);
signal(SIGALRM, handler);
kill(getpid(), SIGALRM);
return 0x23;
}
void *getip(int pid)
{
return (void*)ptrace(PTRACE_PEEKUSER, pid,
offsetof(struct user, regs.rip), 0);
}
int main(void)
{
int pid, status;
pid = fork();
if (!pid)
return child();
assert(wait(&status) == pid);
assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGALRM);
assert(ptrace(PTRACE_SINGLESTEP, pid, 0, SIGALRM) == 0);
assert(wait(&status) == pid);
assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP);
assert((getip(pid) - (void*)handler) == 0);
assert(ptrace(PTRACE_SINGLESTEP, pid, 0, SIGALRM) == 0);
assert(wait(&status) == pid);
assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP);
assert((getip(pid) - (void*)handler) == 1);
assert(ptrace(PTRACE_CONT, pid, 0,0) == 0);
assert(wait(&status) == pid);
assert(WIFEXITED(status) && WEXITSTATUS(status) == 0x23);
return 0;
}
The last assert() fails because PTRACE_CONT wrongly triggers
another single-step and X86_EFLAGS_TF can't be cleared by
debugger until the tracee does sys_rt_sigreturn().
Change handle_signal() to do user_disable_single_step() if
stepping, we do not need to preserve TIF_SINGLESTEP because we
are going to do ptrace_notify(), and it is simply wrong to leak
this bit.
While at it, change the comment to explain why we also need to
clear TF unconditionally after setup_rt_frame().
Note: in the longer term we should probably change
setup_sigcontext() to use get_flags() and then just remove this
user_disable_single_step(). And, the state of TIF_FORCED_TF can
be wrong after restore_sigcontext() which can set/clear TF, this
needs another fix.
This fix fixes the 'single_step_syscall_32' testcase in
the x86 testsuite:
Before:
~/linux/tools/testing/selftests/x86> ./single_step_syscall_32
[RUN] Set TF and check nop
[OK] Survived with TF set and 9 traps
[RUN] Set TF and check int80
[OK] Survived with TF set and 9 traps
[RUN] Set TF and check a fast syscall
[WARN] Hit 10000 SIGTRAPs with si_addr 0xf7789cc0, ip 0xf7789cc0
Trace/breakpoint trap (core dumped)
After:
~/linux/linux/tools/testing/selftests/x86> ./single_step_syscall_32
[RUN] Set TF and check nop
[OK] Survived with TF set and 9 traps
[RUN] Set TF and check int80
[OK] Survived with TF set and 9 traps
[RUN] Set TF and check a fast syscall
[OK] Survived with TF set and 39 traps
[RUN] Fast syscall with TF cleared
[OK] Nothing unexpected happened
Reported-by: Evan Teran <eteran@alum.rit.edu>
Reported-by: Pedro Alves <palves@redhat.com>
Tested-by: Andres Freund <andres@anarazel.de>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
[ Added x86 self-test info. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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