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Subject[tip:x86/urgent] x86/unwind: Handle NULL pointer calls better in frame unwinder
Commit-ID:  f4f34e1b82eb4219d8eaa1c7e2e17ca219a6a2b5
Gitweb: https://git.kernel.org/tip/f4f34e1b82eb4219d8eaa1c7e2e17ca219a6a2b5
Author: Jann Horn <jannh@google.com>
AuthorDate: Fri, 1 Mar 2019 04:12:00 +0100
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Wed, 6 Mar 2019 23:03:26 +0100

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

---
arch/x86/include/asm/unwind.h | 6 ++++++
arch/x86/kernel/unwind_frame.c | 25 ++++++++++++++++++++++---
2 files changed, 28 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/unwind.h b/arch/x86/include/asm/unwind.h
index 1f86e1b0a5cd..499578f7e6d7 100644
--- a/arch/x86/include/asm/unwind.h
+++ b/arch/x86/include/asm/unwind.h
@@ -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;
diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
index 3dc26f95d46e..9b9fd4826e7a 100644
--- a/arch/x86/kernel/unwind_frame.c
+++ b/arch/x86/kernel/unwind_frame.c
@@ -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);
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