x86/ptrace: Fix the TIF_FORCED_TF logic in handle_signal()
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>
parent
0a15584d
Please register or sign in to comment