If the value of the link register is not correct (tail call from asm
that didn't set it, stack corruption, memory no longer mapped), then
using it for an address calculation may trigger an exception. Without a
fixup handler, this will lead to a panic, which will unwind, which will
trigger the fault repeatedly in an infinite loop.
We don't observe such failures currently, but we have. Just to be safe,
add a fixup handler here so that at least we don't have an infinite
loop.
Cc: stable@xxxxxxxxxxxxxxx
Fixes: commit 6dc5fd93b2f1 ("ARM: 8900/1: UNWINDER_FRAME_POINTER implementation for Clang")
Reported-by: Miles Chen <miles.chen@xxxxxxxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
arch/arm/lib/backtrace-clang.S | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/arch/arm/lib/backtrace-clang.S b/arch/arm/lib/backtrace-clang.S
index 5388ac664c12..40eb2215eaf4 100644
--- a/arch/arm/lib/backtrace-clang.S
+++ b/arch/arm/lib/backtrace-clang.S
@@ -146,7 +146,7 @@ for_each_frame: tst frame, mask @ Check for address exceptions
tst sv_lr, #0 @ If there's no previous lr,
beq finished_setup @ we're done.
- ldr r0, [sv_lr, #-4] @ get call instruction
+prev_call: ldr r0, [sv_lr, #-4] @ get call instruction
ldr r3, .Lopcode+4
and r2, r3, r0 @ is this a bl call
teq r2, r3
@@ -206,6 +206,13 @@ finished_setup:
mov r2, frame
bl printk
no_frame: ldmfd sp!, {r4 - r9, fp, pc}
+/*
+ * Accessing the address pointed to by the link register triggered an
+ * exception, don't try to unwind through it.
+ */
+bad_lr: mov sv_fp, #0
+ mov sv_lr, #0
+ b finished_setup
ENDPROC(c_backtrace)
.pushsection __ex_table,"a"
.align 3
@@ -214,6 +221,7 @@ ENDPROC(c_backtrace)
.long 1003b, 1006b
.long 1004b, 1006b
.long 1005b, 1006b
+ .long prev_call, bad_lr
.popsection
.Lbad: .asciz "%sBacktrace aborted due to bad frame pointer <%p>\n"
--
2.28.0.163.g6104cc2f0b6-goog
