Patch "riscv: jump_label: Fixup unaligned arch_static_branch function" has been added to the 6.2-stable tree

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This is a note to let you know that I've just added the patch titled

    riscv: jump_label: Fixup unaligned arch_static_branch function

to the 6.2-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     riscv-jump_label-fixup-unaligned-arch_static_branch-function.patch
and it can be found in the queue-6.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.


>From 9ddfc3cd806081ce1f6c9c2f988cbb031f35d28f Mon Sep 17 00:00:00 2001
From: Andy Chiu <andy.chiu@xxxxxxxxxx>
Date: Mon, 6 Feb 2023 04:04:40 -0500
Subject: riscv: jump_label: Fixup unaligned arch_static_branch function

From: Andy Chiu <andy.chiu@xxxxxxxxxx>

commit 9ddfc3cd806081ce1f6c9c2f988cbb031f35d28f upstream.

Runtime code patching must be done at a naturally aligned address, or we
may execute on a partial instruction.

We have encountered problems traced back to static jump functions during
the test. We switched the tracer randomly for every 1~5 seconds on a
dual-core QEMU setup and found the kernel sucking at a static branch
where it jumps to itself.

The reason is that the static branch was 2-byte but not 4-byte aligned.
Then, the kernel would patch the instruction, either J or NOP, with two
half-word stores if the machine does not have efficient unaligned
accesses. Thus, moments exist where half of the NOP mixes with the other
half of the J when transitioning the branch. In our particular case, on
a little-endian machine, the upper half of the NOP was mixed with the
lower part of the J when enabling the branch, resulting in a jump that
jumped to itself. Conversely, it would result in a HINT instruction when
disabling the branch, but it might not be observable.

ARM64 does not have this problem since all instructions must be 4-byte
aligned.

Fixes: ebc00dde8a97 ("riscv: Add jump-label implementation")
Link: https://lore.kernel.org/linux-riscv/20220913094252.3555240-6-andy.chiu@xxxxxxxxxx/
Reviewed-by: Greentime Hu <greentime.hu@xxxxxxxxxx>
Signed-off-by: Andy Chiu <andy.chiu@xxxxxxxxxx>
Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20230206090440.1255001-1-guoren@xxxxxxxxxx
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Palmer Dabbelt <palmer@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/riscv/include/asm/jump_label.h |    2 ++
 1 file changed, 2 insertions(+)

--- a/arch/riscv/include/asm/jump_label.h
+++ b/arch/riscv/include/asm/jump_label.h
@@ -18,6 +18,7 @@ static __always_inline bool arch_static_
 					       const bool branch)
 {
 	asm_volatile_goto(
+		"	.align		2			\n\t"
 		"	.option push				\n\t"
 		"	.option norelax				\n\t"
 		"	.option norvc				\n\t"
@@ -39,6 +40,7 @@ static __always_inline bool arch_static_
 						    const bool branch)
 {
 	asm_volatile_goto(
+		"	.align		2			\n\t"
 		"	.option push				\n\t"
 		"	.option norelax				\n\t"
 		"	.option norvc				\n\t"


Patches currently in stable-queue which might be from andy.chiu@xxxxxxxxxx are

queue-6.2/riscv-jump_label-fixup-unaligned-arch_static_branch-function.patch
queue-6.2/riscv-ftrace-fixup-panic-by-disabling-preemption.patch



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