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x86/ptrace: Stop setting TS_COMPAT in ptrace code

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Setting TS_COMPAT in ptrace is wrong: if we happen to do it during
syscall entry, then we'll confuse seccomp and audit.  (The former
isn't a security problem: seccomp is currently entirely insecure if a
malicious ptracer is attached.)  As a minimal fix, this patch adds a
new flag TS_I386_REGS_POKED that handles the ptrace special case.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
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: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Pedro Alves <palves@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/5383ebed38b39fa37462139e337aff7f2314d1ca.1469599803.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Andy Lutomirski 2016-07-26 23:12:22 -07:00 committed by Ingo Molnar
parent 37e13a1ebe
commit 609c19a385
5 changed files with 42 additions and 13 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

6
arch/x86/entry/common.c
View file

@ -270,8 +270,12 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
* handling, because syscall restart has a fixup for compat
* syscalls. The fixup is exercised by the ptrace_syscall_32
* selftest.
*
* We also need to clear TS_REGS_POKED_I386: the 32-bit tracer
* special case only applies after poking regs and before the
* very next return to user mode.
*/
ti->status &= ~TS_COMPAT;
ti->status &= ~(TS_COMPAT|TS_I386_REGS_POKED);
#endif
user_enter_irqoff();

5
arch/x86/include/asm/syscall.h
View file

@ -60,7 +60,7 @@ static inline long syscall_get_error(struct task_struct *task,
* TS_COMPAT is set for 32-bit syscall entries and then
* remains set until we return to user mode.
*/
if (task_thread_info(task)->status & TS_COMPAT)
if (task_thread_info(task)->status & (TS_COMPAT|TS_I386_REGS_POKED))
/*
* Sign-extend the value so (int)-EFOO becomes (long)-EFOO
* and will match correctly in comparisons.
@ -239,9 +239,6 @@ static inline int syscall_get_arch(void)
* TS_COMPAT is set for 32-bit syscall entry and then
* remains set until we return to user mode.
*
* TIF_IA32 tasks should always have TS_COMPAT set at
* system call time.
*
* x32 tasks should be considered AUDIT_ARCH_X86_64.
*/
if (task_thread_info(current)->status & TS_COMPAT)

3
arch/x86/include/asm/thread_info.h
View file

@ -219,6 +219,9 @@ static inline unsigned long current_stack_pointer(void)
* have to worry about atomic accesses.
*/
#define TS_COMPAT 0x0002 /* 32bit syscall active (64BIT)*/
#ifdef CONFIG_COMPAT
#define TS_I386_REGS_POKED 0x0004 /* regs poked by 32-bit ptracer */
#endif
#define TS_RESTORE_SIGMASK 0x0008 /* restore signal mask in do_signal() */
#ifndef __ASSEMBLY__

15
arch/x86/kernel/ptrace.c
View file

@ -923,15 +923,18 @@ static int putreg32(struct task_struct *child, unsigned regno, u32 value)
case offsetof(struct user32, regs.orig_eax):
/*
* A 32-bit debugger setting orig_eax means to restore
* the state of the task restarting a 32-bit syscall.
* Make sure we interpret the -ERESTART* codes correctly
* in case the task is not actually still sitting at the
* exit from a 32-bit syscall with TS_COMPAT still set.
* Warning: bizarre corner case fixup here. A 32-bit
* debugger setting orig_eax to -1 wants to disable
* syscall restart. Make sure that the syscall
* restart code sign-extends orig_ax. Also make sure
* we interpret the -ERESTART* codes correctly if
* loaded into regs->ax in case the task is not
* actually still sitting at the exit from a 32-bit
* syscall with TS_COMPAT still set.
*/
regs->orig_ax = value;
if (syscall_get_nr(child, regs) >= 0)
task_thread_info(child)->status |= TS_COMPAT;
task_thread_info(child)->status |= TS_I386_REGS_POKED;
break;
case offsetof(struct user32, regs.eflags):

26
arch/x86/kernel/signal.c
View file

@ -760,8 +760,30 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
static inline unsigned long get_nr_restart_syscall(const struct pt_regs *regs)
{
#ifdef CONFIG_X86_64
if (in_ia32_syscall())
/*
* This function is fundamentally broken as currently
* implemented.
*
* The idea is that we want to trigger a call to the
* restart_block() syscall and that we want in_ia32_syscall(),
* in_x32_syscall(), etc. to match whatever they were in the
* syscall being restarted. We assume that the syscall
* instruction at (regs->ip - 2) matches whatever syscall
* instruction we used to enter in the first place.
*
* The problem is that we can get here when ptrace pokes
* syscall-like values into regs even if we're not in a syscall
* at all.
*
* For now, we maintain historical behavior and guess based on
* stored state. We could do better by saving the actual
* syscall arch in restart_block or (with caveats on x32) by
* checking if regs->ip points to 'int $0x80'. The current
* behavior is incorrect if a tracer has a different bitness
* than the tracee.
*/
#ifdef CONFIG_IA32_EMULATION
if (current_thread_info()->status & (TS_COMPAT|TS_I386_REGS_POKED))
return __NR_ia32_restart_syscall;
#endif
#ifdef CONFIG_X86_X32_ABI