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Subject[PATCH 1/1] ptrace: x86: stepping in a signal handler leaks X86_EFLAGS_TF
See https://bugzilla.kernel.org/show_bug.cgi?id=16061

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 (based on the excellent report from Evan):

#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;
}
Change handle_signal() to clear TIF_SINGLESTEP as well. We cleared
X86_EFLAGS_TF and TIF_FORCED_TF, we are not going to return to user-mode
if TIF_SINGLESTEP was set, and we already passed syscall_trace_leave().
We are going to sleep until the next ptrace_resume() which should set
these flags correctly if needed.

Note: this is the most simple fix now, most probably we need more
changes/cleanups on top of this patch.

Reported-by: Evan Teran <eteran@alum.rit.edu>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
arch/x86/kernel/signal.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- 34-rc1/arch/x86/kernel/signal.c~BZ16061_TEMPORARY_FIX 2010-06-06 16:22:55.000000000 +0200
+++ 34-rc1/arch/x86/kernel/signal.c 2010-06-06 16:47:55.000000000 +0200
@@ -749,7 +749,7 @@ handle_signal(unsigned long sig, siginfo
spin_unlock_irq(&current->sighand->siglock);

tracehook_signal_handler(sig, info, ka, regs,
- test_thread_flag(TIF_SINGLESTEP));
+ test_and_clear_thread_flag(TIF_SINGLESTEP));

return 0;
}


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