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x86/unwind: Handle NULL pointer calls better in frame unwinder
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
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2 changed files with 28 additions and 3 deletions
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@ -23,6 +23,12 @@ struct unwind_state {
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#elif defined(CONFIG_UNWINDER_FRAME_POINTER)
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bool got_irq;
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unsigned long *bp, *orig_sp, ip;
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/*
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* If non-NULL: The current frame is incomplete and doesn't contain a
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* valid BP. When looking for the next frame, use this instead of the
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* non-existent saved BP.
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*/
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unsigned long *next_bp;
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struct pt_regs *regs;
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#else
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unsigned long *sp;
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@ -320,10 +320,14 @@ bool unwind_next_frame(struct unwind_state *state)
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}
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/* Get the next frame pointer: */
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if (state->regs)
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if (state->next_bp) {
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next_bp = state->next_bp;
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state->next_bp = NULL;
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} else if (state->regs) {
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next_bp = (unsigned long *)state->regs->bp;
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else
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} else {
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next_bp = (unsigned long *)READ_ONCE_TASK_STACK(state->task, *state->bp);
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}
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/* Move to the next frame if it's safe: */
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if (!update_stack_state(state, next_bp))
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@ -398,6 +402,21 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
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bp = get_frame_pointer(task, regs);
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/*
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* If we crash with IP==0, the last successfully executed instruction
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* was probably an indirect function call with a NULL function pointer.
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* That means that SP points into the middle of an incomplete frame:
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* *SP is a return pointer, and *(SP-sizeof(unsigned long)) is where we
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* would have written a frame pointer if we hadn't crashed.
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* Pretend that the frame is complete and that BP points to it, but save
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* the real BP so that we can use it when looking for the next frame.
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*/
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if (regs && regs->ip == 0 &&
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(unsigned long *)kernel_stack_pointer(regs) >= first_frame) {
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state->next_bp = bp;
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bp = ((unsigned long *)kernel_stack_pointer(regs)) - 1;
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}
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/* Initialize stack info and make sure the frame data is accessible: */
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get_stack_info(bp, state->task, &state->stack_info,
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&state->stack_mask);
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@ -410,7 +429,7 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
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*/
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while (!unwind_done(state) &&
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(!on_stack(&state->stack_info, first_frame, sizeof(long)) ||
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state->bp < first_frame))
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(state->next_bp == NULL && state->bp < first_frame)))
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unwind_next_frame(state);
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}
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EXPORT_SYMBOL_GPL(__unwind_start);
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