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x86/unwind: Handle NULL pointer calls better in frame unwinder

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When the frame unwinder is invoked for an oops caused by a call to NULL, it
currently skips the parent function because BP still points to the parent's
stack frame; the (nonexistent) current function only has the first half of
a stack frame, and BP doesn't point to it yet.

Add a special case for IP==0 that calculates a fake BP from SP, then uses
the real BP for the next frame.

Note that this handles first_frame specially: Return information about the
parent function as long as the saved IP is >=first_frame, even if the fake
BP points below it.

With an artificially-added NULL call in prctl_set_seccomp(), before this
patch, the trace is:

Call Trace:
 ? prctl_set_seccomp+0x3a/0x50
 __x64_sys_prctl+0x457/0x6f0
 ? __ia32_sys_prctl+0x750/0x750
 do_syscall_64+0x72/0x160
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

After this patch, the trace is:

Call Trace:
 prctl_set_seccomp+0x3a/0x50
 __x64_sys_prctl+0x457/0x6f0
 ? __ia32_sys_prctl+0x750/0x750
 do_syscall_64+0x72/0x160
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: syzbot <syzbot+ca95b2b7aef9e7cbd6ab@syzkaller.appspotmail.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Michal Marek <michal.lkml@markovi.net>
Cc: linux-kbuild@vger.kernel.org
Link: https://lkml.kernel.org/r/20190301031201.7416-1-jannh@google.com
This commit is contained in:
Jann Horn 2019-03-01 04:12:00 +01:00 committed by Thomas Gleixner
parent f76a16adc4
commit f4f34e1b82
2 changed files with 28 additions and 3 deletions
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6
arch/x86/include/asm/unwind.h
View File

@ -23,6 +23,12 @@ struct unwind_state {
#elif defined(CONFIG_UNWINDER_FRAME_POINTER)
bool got_irq;
unsigned long *bp, *orig_sp, ip;
/*
* If non-NULL: The current frame is incomplete and doesn't contain a
* valid BP. When looking for the next frame, use this instead of the
* non-existent saved BP.
*/
unsigned long *next_bp;
struct pt_regs *regs;
#else
unsigned long *sp;

25
arch/x86/kernel/unwind_frame.c
View File

@ -320,10 +320,14 @@ bool unwind_next_frame(struct unwind_state *state)
}
/* Get the next frame pointer: */
if (state->regs)
if (state->next_bp) {
next_bp = state->next_bp;
state->next_bp = NULL;
} else if (state->regs) {
next_bp = (unsigned long *)state->regs->bp;
else
} else {
next_bp = (unsigned long *)READ_ONCE_TASK_STACK(state->task, *state->bp);
}
/* Move to the next frame if it's safe: */
if (!update_stack_state(state, next_bp))
@ -398,6 +402,21 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
bp = get_frame_pointer(task, regs);
/*
* If we crash with IP==0, the last successfully executed instruction
* was probably an indirect function call with a NULL function pointer.
* That means that SP points into the middle of an incomplete frame:
* *SP is a return pointer, and *(SP-sizeof(unsigned long)) is where we
* would have written a frame pointer if we hadn't crashed.
* Pretend that the frame is complete and that BP points to it, but save
* the real BP so that we can use it when looking for the next frame.
*/
if (regs && regs->ip == 0 &&
(unsigned long *)kernel_stack_pointer(regs) >= first_frame) {
state->next_bp = bp;
bp = ((unsigned long *)kernel_stack_pointer(regs)) - 1;
}
/* Initialize stack info and make sure the frame data is accessible: */
get_stack_info(bp, state->task, &state->stack_info,
&state->stack_mask);
@ -410,7 +429,7 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
*/
while (!unwind_done(state) &&
(!on_stack(&state->stack_info, first_frame, sizeof(long)) ||
state->bp < first_frame))
(state->next_bp == NULL && state->bp < first_frame)))
unwind_next_frame(state);
}
EXPORT_SYMBOL_GPL(__unwind_start);