arm64: tags: Preserve tags for addresses translated via TTBR1

Sign-extending TTBR1 addresses when converting to an untagged address
breaks the documented POSIX semantics for mlock() in some obscure error
cases where we end up returning -EINVAL instead of -ENOMEM as a direct
result of rewriting the upper address bits.

Rework the untagged_addr() macro to preserve the upper address bits for
TTBR1 addresses and only clear the tag bits for user addresses. This
matches the behaviour of the 'clear_address_tag' assembly macro, so
rename that and align the implementations at the same time so that they
use the same instruction sequences for the tag manipulation.

Link: https://lore.kernel.org/stable/20191014162651.GF19200@arrakis.emea.arm.com/
Reported-by: Jan Stancek <jstancek@redhat.com>
Tested-by: Jan Stancek <jstancek@redhat.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Tested-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/asm/asm-uaccess.h

@@ -78,10 +78,9 @@ alternative_else_nop_endif
 /*
  * Remove the address tag from a virtual address, if present.
  */
-	.macro	clear_address_tag, dst, addr
-	tst	\addr, #(1 << 55)
-	bic	\dst, \addr, #(0xff << 56)
-	csel	\dst, \dst, \addr, eq
+	.macro	untagged_addr, dst, addr
+	sbfx	\dst, \addr, #0, #56
+	and	\dst, \dst, \addr
 	.endm
 
 #endif

arch/arm64/include/asm/memory.h

@@ -215,12 +215,18 @@ static inline unsigned long kaslr_offset(void)
  * up with a tagged userland pointer. Clear the tag to get a sane pointer to
  * pass on to access_ok(), for instance.
  */
-#define untagged_addr(addr)	\
+#define __untagged_addr(addr)	\
 	((__force __typeof__(addr))sign_extend64((__force u64)(addr), 55))
 
+#define untagged_addr(addr)	({					\
+	u64 __addr = (__force u64)addr;					\
+	__addr &= __untagged_addr(__addr);				\
+	(__force __typeof__(addr))__addr;				\
+})
+
 #ifdef CONFIG_KASAN_SW_TAGS
 #define __tag_shifted(tag)	((u64)(tag) << 56)
-#define __tag_reset(addr)	untagged_addr(addr)
+#define __tag_reset(addr)	__untagged_addr(addr)
 #define __tag_get(addr)		(__u8)((u64)(addr) >> 56)
 #else
 #define __tag_shifted(tag)	0UL

arch/arm64/kernel/entry.S

@@ -604,7 +604,7 @@ el1_da:
 	 */
 	mrs	x3, far_el1
 	inherit_daif	pstate=x23, tmp=x2
-	clear_address_tag x0, x3
+	untagged_addr	x0, x3
 	mov	x2, sp				// struct pt_regs
 	bl	do_mem_abort
 
@@ -808,7 +808,7 @@ el0_da:
 	mrs	x26, far_el1
 	ct_user_exit_irqoff
 	enable_daif
-	clear_address_tag x0, x26
+	untagged_addr	x0, x26
 	mov	x1, x25
 	mov	x2, sp
 	bl	do_mem_abort