arm64: compat: fix stxr failure case in SWP emulation

If the STXR instruction fails in the SWP emulation code, we leave *data
overwritten with the loaded value, therefore corrupting the data written
by a subsequent, successful attempt.

This patch re-jigs the code so that we only write back to *data once we
know that the update has happened.

Cc: <stable@vger.kernel.org>
Fixes: bd35a4adc413 ("arm64: Port SWP/SWPB emulation support from arm")
Reported-by: Shengjiu Wang <shengjiu.wang@freescale.com>
Reported-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

diff --git a/arch/arm64/kernel/armv8_deprecated.c b/arch/arm64/kernel/armv8_deprecated.c
index bcee7ab..937f5e5 100644 (file)
--- a/arch/arm64/kernel/armv8_deprecated.c
+++ b/arch/arm64/kernel/armv8_deprecated.c
@@ -284,21 +284,23 @@ static void register_insn_emulation_sysctl(struct ctl_table *table)
        __asm__ __volatile__(                                   \
        ALTERNATIVE("nop", SET_PSTATE_PAN(0), ARM64_HAS_PAN,    \
                    CONFIG_ARM64_PAN)                           \
-       "       mov             %w2, %w1\n"                     \
-       "0:     ldxr"B"         %w1, [%3]\n"                    \
-       "1:     stxr"B"         %w0, %w2, [%3]\n"               \
+       "0:     ldxr"B"         %w2, [%3]\n"                    \
+       "1:     stxr"B"         %w0, %w1, [%3]\n"               \
        "       cbz             %w0, 2f\n"                      \
        "       mov             %w0, %w4\n"                     \
+       "       b               3f\n"                           \
        "2:\n"                                                  \
+       "       mov             %w1, %w2\n"                     \
+       "3:\n"                                                  \
        "       .pushsection     .fixup,\"ax\"\n"               \
        "       .align          2\n"                            \
-       "3:     mov             %w0, %w5\n"                     \
-       "       b               2b\n"                           \
+       "4:     mov             %w0, %w5\n"                     \
+       "       b               3b\n"                           \
        "       .popsection"                                    \
        "       .pushsection     __ex_table,\"a\"\n"            \
        "       .align          3\n"                            \
-       "       .quad           0b, 3b\n"                       \
-       "       .quad           1b, 3b\n"                       \
+       "       .quad           0b, 4b\n"                       \
+       "       .quad           1b, 4b\n"                       \
        "       .popsection\n"                                  \
        ALTERNATIVE("nop", SET_PSTATE_PAN(1), ARM64_HAS_PAN,    \
                CONFIG_ARM64_PAN)                               \