[PATCH AUTOSEL 5.6 300/606] ARM: uaccess: fix DACR mismatch with nested exceptions

Sasha Levin
2020-06-09T00:19:26Z

From: Russell King <rmk+kernel@armlinux.org.uk>

[ Upstream commit 71f8af1110101facfad68989ff91f88f8e2c3e22 ]

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 <tomas.paukrt@advantech.cz>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/arm/include/asm/uaccess-asm.h | 25 ++++++++++++++++++++-----
1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/arch/arm/include/asm/uaccess-asm.h b/arch/arm/include/asm/uaccess-asm.h
index e46468b91eaa..907571fd05c6 100644
--- a/arch/arm/include/asm/uaccess-asm.h
+++ b/arch/arm/include/asm/uaccess-asm.h
@@ -67,15 +67,21 @@
#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]
@@ -84,8 +90,17 @@
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

--
2.25.1