[PATCH] arm64: fix strlen() with CONFIG_KASAN_HW_TAGS

[Mark Rutland]

When the kernel is built with CONFIG_KASAN_HW_TAGS and the CPU supports
MTE, memory accesses are checked at 16-byte granularity, and
out-of-bounds accesses can result in tag check faults. Our current
implementation of strlen() makes unaligned 16-byte accesses (within a
naturally aligned 4096-byte window), and can trigger tag check faults.

This can be seen at boot time, e.g.

| BUG: KASAN: invalid-access in __pi_strlen+0x14/0x150
| Read at addr f4ff0000c0028300 by task swapper/0/0
| Pointer tag: [f4], memory tag: [fe]
|
| CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.13.0-09550-g03c2813535a2-dirty #20
| Hardware name: linux,dummy-virt (DT)
| Call trace:
|  dump_backtrace+0x0/0x1b0
|  show_stack+0x1c/0x30
|  dump_stack_lvl+0x68/0x84
|  print_address_description+0x7c/0x2b4
|  kasan_report+0x138/0x38c
|  __do_kernel_fault+0x190/0x1c4
|  do_tag_check_fault+0x78/0x90
|  do_mem_abort+0x44/0xb4
|  el1_abort+0x40/0x60
|  el1h_64_sync_handler+0xb0/0xd0
|  el1h_64_sync+0x78/0x7c
|  __pi_strlen+0x14/0x150
|  __register_sysctl_table+0x7c4/0x890
|  register_leaf_sysctl_tables+0x1a4/0x210
|  register_leaf_sysctl_tables+0xc8/0x210
|  __register_sysctl_paths+0x22c/0x290
|  register_sysctl_table+0x2c/0x40
|  sysctl_init+0x20/0x30
|  proc_sys_init+0x3c/0x48
|  proc_root_init+0x80/0x9c
|  start_kernel+0x640/0x69c
|  __primary_switched+0xc0/0xc8

To fix this, we can reduce the (strlen-internal) MIN_PAGE_SIZE to 16
bytes when CONFIG_KASAN_HW_TAGS is selected. This will cause strlen() to
align the base pointer downwards to a 16-byte boundary, and to discard
the additional prefix bytes without counting them. All subsequent
accesses will be 16-byte aligned 16-byte LDPs. While the comments say
the body of the loop will access 32 bytes, this is performed as two
16-byte acceses, with the second made only if the first did not
encounter a NUL byte, so the body of the loop will not over-read across
a 16-byte boundary.

No other string routines are affected. The other str*() routines will
not make any access which straddles a 16-byte boundary, and the mem*()
routines will only make acceses which straddle a 16-byte boundary when
which is entirely within the bounds of the relevant base and size
arguments.

Fixes: 325a1de81287a3d4 (“arm64: Import updated version of Cortex Strings' strlen”)
Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Alexander Potapenko <glider at google.com
Cc: Andrey Konovalov <andreyknvl at gmail.com>
Cc: Andrey Ryabinin <ryabinin.a.a at gmail.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Dmitry Vyukov <dvyukov at google.com>
Cc: Marco Elver <elver at google.com>
Cc: Robin Murphy <robin.murphy at arm.com>
Cc: Will Deacon <will at kernel.org>
---
 arch/arm64/lib/strlen.S | 10 ++++++++++
 1 file changed, 10 insertions(+)

Note: to build-test this, you'll also need to apply a fix from Marco [1], which
is on its way to mainline via akpm. I've locally aplied that, and tested this
patch with a recent build of QEMU with MTE enabled.

Mark.

diff --git a/arch/arm64/lib/strlen.S b/arch/arm64/lib/strlen.S
index 35fbdb7d6e1a..1648790e91b3 100644
--- a/arch/arm64/lib/strlen.S
+++ b/arch/arm64/lib/strlen.S
@@ -8,6 +8,7 @@
 
 #include <linux/linkage.h>
 #include <asm/assembler.h>
+#include <asm/mte-def.h>
 
 /* Assumptions:
  *
@@ -42,7 +43,16 @@
 #define REP8_7f 0x7f7f7f7f7f7f7f7f
 #define REP8_80 0x8080808080808080
 
+/*
+ * When KASAN_HW_TAGS is in use, memory is checked at MTE_GRANULE_SIZE
+ * (16-byte) granularity, and we must ensure that no access straddles this
+ * alignment boundary.
+ */
+#ifdef CONFIG_KASAN_HW_TAGS
+#define MIN_PAGE_SIZE MTE_GRANULE_SIZE
+#else
 #define MIN_PAGE_SIZE 4096
+#endif
 
 	/* Since strings are short on average, we check the first 16 bytes
 	   of the string for a NUL character.  In order to do an unaligned ldp
-- 
2.11.0