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

diff --git a/arch/x86/include/asm/unwind.h b/arch/x86/include/asm/unwind.h
index 1f86e1b..499578f 100644 (file)
--- 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 3dc26f9..9b9fd48 100644 (file)
--- 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);