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

#endif
}

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

@@ -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
 	ret
 ENDPROC(__clear_user)

Thanks for this, it's been like this for a while. Applied.

Catalin

Thanks Catalin, if it's not too late, I forgot to add a

for the initial report of the panic.

regards, Kyle

I'll add this, I haven't pushed it out yet.

Catalin