ARM: uaccess: fix DACR mismatch with nested exceptions

Tomas Paukrt reports that his SAM9X60 based system (ARM926, ARMv5TJ)
fails to fix up alignment faults, eventually resulting in a kernel
oops.

The problem occurs when using CONFIG_CPU_USE_DOMAINS with commit
e6978e4bf181 ("ARM: save and reset the address limit when entering an
exception"). This is because the address limit is set back to
TASK_SIZE on exception entry, and, although it is restored on exception
exit, the domain register is not.

Hence, this sequence can occur:

interrupt
pt_regs->addr_limit = addr_limit // USER_DS
addr_limit = USER_DS
alignment exception
__probe_kernel_read()
old_fs = get_fs() // USER_DS
set_fs(KERNEL_DS)
addr_limit = KERNEL_DS
dacr.kernel = DOMAIN_MANAGER
interrupt
pt_regs->addr_limit = addr_limit // KERNEL_DS
addr_limit = USER_DS
alignment exception
__probe_kernel_read()
old_fs = get_fs() // USER_DS
set_fs(KERNEL_DS)
addr_limit = KERNEL_DS
dacr.kernel = DOMAIN_MANAGER
...
set_fs(old_fs)
addr_limit = USER_DS
dacr.kernel = DOMAIN_CLIENT
...
addr_limit = pt_regs->addr_limit // KERNEL_DS
interrupt returns

At this point, addr_limit is correctly restored to KERNEL_DS for
__probe_kernel_read() to continue execution, but dacr.kernel is not,
it has been reset by the set_fs(old_fs) to DOMAIN_CLIENT.

This would not have happened prior to the mentioned commit, because
addr_limit would remain KERNEL_DS, so get_fs() would have returned
KERNEL_DS, and so would correctly nest.

This commit fixes the problem by also saving the DACR on exception
entry if either CONFIG_CPU_SW_DOMAIN_PAN or CONFIG_CPU_USE_DOMAINS are
enabled, and resetting the DACR appropriately on exception entry to
match addr_limit and PAN settings.

Fixes: e6978e4bf181 ("ARM: save and reset the address limit when entering an exception")
Reported-by: Tomas Paukrt <[e-mail]>
Signed-off-by: Russell King <[e-mail]>

arch/arm/include/asm/uaccess-asm.h

···
 #endif
 	.endm
 
-#ifdef CONFIG_CPU_SW_DOMAIN_PAN
+#if defined(CONFIG_CPU_SW_DOMAIN_PAN) || defined(CONFIG_CPU_USE_DOMAINS)
 #define DACR(x...)	x
 #else
 #define DACR(x...)
 #endif
 
 	/*
-	 * Save the address limit on entry to a privileged exception and
-	 * if using PAN, save and disable usermode access.
+	 * Save the address limit on entry to a privileged exception.
+	 *
+	 * If we are using the DACR for kernel access by the user accessors
+	 * (CONFIG_CPU_USE_DOMAINS=y), always reset the DACR kernel domain
+	 * back to client mode, whether or not \disable is set.
+	 *
+	 * If we are using SW PAN, set the DACR user domain to no access
+	 * if \disable is set.
 	 */
 	.macro	uaccess_entry, tsk, tmp0, tmp1, tmp2, disable
 	ldr	\tmp1, [\tsk, #TI_ADDR_LIMIT]
···
  DACR(	mrc	p15, 0, \tmp0, c3, c0, 0)
  DACR(	str	\tmp0, [sp, #SVC_DACR])
 	str	\tmp1, [sp, #SVC_ADDR_LIMIT]
-	.if \disable
-	uaccess_disable \tmp0
+	.if \disable && IS_ENABLED(CONFIG_CPU_SW_DOMAIN_PAN)
+	/* kernel=client, user=no access */
+	mov	\tmp2, #DACR_UACCESS_DISABLE
+	mcr	p15, 0, \tmp2, c3, c0, 0
+	instr_sync
+	.elseif IS_ENABLED(CONFIG_CPU_USE_DOMAINS)
+	/* kernel=client */
+	bic	\tmp2, \tmp0, #domain_mask(DOMAIN_KERNEL)
+	orr	\tmp2, \tmp2, #domain_val(DOMAIN_KERNEL, DOMAIN_CLIENT)
+	mcr	p15, 0, \tmp2, c3, c0, 0
+	instr_sync
 	.endif
 	.endm