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Subject[PATCH] arm64: __clear_user: handle exceptions on strb
ARM64 currently doesn't fix up faults on the single-byte (strb) case of
__clear_user... which means that we can cause a nasty kernel panic as an
ordinary user with any multiple PAGE_SIZE+1 read from /dev/zero.
i.e.: dd if=/dev/zero of=foo ibs=1 count=1 (or ibs=65537, etc.)

This is a pretty obscure bug in the general case since we'll only
__do_kernel_fault (since there's no extable entry for pc) if the
mmap_sem is contended. However, with CONFIG_DEBUG_VM enabled, we'll
always fault.

if (!down_read_trylock(&mm->mmap_sem)) {
if (!user_mode(regs) && !search_exception_tables(regs->pc))
goto no_context;
} else {
* The above down_read_trylock() might have succeeded in
* which
* case, we'll have missed the might_sleep() from
* down_read().
if (!user_mode(regs) && !search_exception_tables(regs->pc))
goto no_context;

Fix that by adding an extable entry for the strb instruction, since it
touches user memory, similar to the other stores in __clear_user.

Signed-off-by: Kyle McMartin <>

--- a/arch/arm64/lib/clear_user.S
+++ b/arch/arm64/lib/clear_user.S
@@ -46,7 +46,7 @@ USER(9f, strh wzr, [x0], #2 )
sub x1, x1, #2
4: adds x1, x1, #1
b.mi 5f
- strb wzr, [x0]
+USER(9f, strb wzr, [x0] )
5: mov x0, #0

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