[PATCH 1/2] x86/unwind: handle NULL pointer calls better in frame unwinder

[Jann Horn]

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: We 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@xxxxxxxxxx>
---
 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);
-- 
2.21.0.352.gf09ad66450-goog