x86/unwind/fp: Fix FP unwinding in ret_from_fork

There have been some reports of "bad bp value" warnings printed by the
frame pointer unwinder:

  WARNING: kernel stack regs at 000000005bac7112 in sh:1014 has bad 'bp' value 0000000000000000

This warning happens when unwinding from an interrupt in
ret_from_fork(). If entry code gets interrupted, the state of the
frame pointer (rbp) may be undefined, which can confuse the unwinder,
resulting in warnings like the above.

There's an in_entry_code() check which normally silences such
warnings for entry code. But in this case, ret_from_fork() is getting
interrupted. It recently got moved out of .entry.text, so the
in_entry_code() check no longer works.

It could be moved back into .entry.text, but that would break the
noinstr validation because of the call to schedule_tail().

Instead, initialize each new task's RBP to point to the task's entry
regs via an encoded frame pointer.  That will allow the unwinder to
reach the end of the stack gracefully.

Fixes: b9f6976bfb94 ("x86/entry/64: Move non entry code into .text section")
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reported-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/f366bbf5a8d02e2318ee312f738112d0af74d16f.1600103007.git.jpoimboe@redhat.com

diff --git a/arch/x86/include/asm/frame.h b/arch/x86/include/asm/frame.h
index 296b346184b27bba25836516f4f24ffa29694a5f..fb42659f6e98860112320225f4bc84e44f43c620 100644 (file)
--- a/arch/x86/include/asm/frame.h
+++ b/arch/x86/include/asm/frame.h
@@ -60,12 +60,26 @@
 #define FRAME_END "pop %" _ASM_BP "\n"
 
 #ifdef CONFIG_X86_64
+
 #define ENCODE_FRAME_POINTER                   \
        "lea 1(%rsp), %rbp\n\t"
+
+static inline unsigned long encode_frame_pointer(struct pt_regs *regs)
+{
+       return (unsigned long)regs + 1;
+}
+
 #else /* !CONFIG_X86_64 */
+
 #define ENCODE_FRAME_POINTER                   \
        "movl %esp, %ebp\n\t"                   \
        "andl $0x7fffffff, %ebp\n\t"
+
+static inline unsigned long encode_frame_pointer(struct pt_regs *regs)
+{
+       return (unsigned long)regs & 0x7fffffff;
+}
+
 #endif /* CONFIG_X86_64 */
 
 #endif /* __ASSEMBLY__ */
@@ -83,6 +97,11 @@
 
 #define ENCODE_FRAME_POINTER
 
+static inline unsigned long encode_frame_pointer(struct pt_regs *regs)
+{
+       return 0;
+}
+
 #endif
 
 #define FRAME_BEGIN

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 13ce616cc7afb7af965fde21772978bee8e07a58..ba4593a913fab3dc6f5b136470c07c02699aeefd 100644 (file)
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -42,6 +42,7 @@
 #include <asm/spec-ctrl.h>
 #include <asm/io_bitmap.h>
 #include <asm/proto.h>
+#include <asm/frame.h>
 
 #include "process.h"
 
@@ -133,7 +134,7 @@ int copy_thread(unsigned long clone_flags, unsigned long sp, unsigned long arg,
        fork_frame = container_of(childregs, struct fork_frame, regs);
        frame = &fork_frame->frame;
 
-       frame->bp = 0;
+       frame->bp = encode_frame_pointer(childregs);
        frame->ret_addr = (unsigned long) ret_from_fork;
        p->thread.sp = (unsigned long) fork_frame;
        p->thread.io_bitmap = NULL;