[PATCH 5.10 057/130] x86/alternative: Try inline spectre_v2=retpoline,amd

Greg Kroah-Hartman
2022-07-12T18:47:17Z

commit bbe2df3f6b6da7848398d55b1311d58a16ec21e4 upstream.

Try and replace retpoline thunk calls with:

LFENCE
CALL *%\reg

for spectre_v2=retpoline,amd.

Specifically, the sequence above is 5 bytes for the low 8 registers,
but 6 bytes for the high 8 registers. This means that unless the
compilers prefix stuff the call with higher registers this replacement
will fail.

Luckily GCC strongly favours RAX for the indirect calls and most (95%+
for defconfig-x86_64) will be converted. OTOH clang strongly favours
R11 and almost nothing gets converted.

Note: it will also generate a correct replacement for the Jcc.d32
case, except unless the compilers start to prefix stuff that, it'll
never fit. Specifically:

Jncc.d8 1f
LFENCE
JMP *%\reg
1:

is 7-8 bytes long, where the original instruction in unpadded form is
only 6 bytes.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20211026120310.359986601@infradead.org
[cascardo: RETPOLINE_AMD was renamed to RETPOLINE_LFENCE]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
arch/x86/kernel/alternative.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -544,6 +544,7 @@ static int emit_indirect(int op, int reg
*
* CALL *%\reg
*
+ * It also tries to inline spectre_v2=retpoline,amd when size permits.
*/
static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
{
@@ -560,7 +561,8 @@ static int patch_retpoline(void *addr, s
/* If anyone ever does: CALL/JMP *%rsp, we're in deep trouble. */
BUG_ON(reg == 4);

- if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
+ if (cpu_feature_enabled(X86_FEATURE_RETPOLINE) &&
+ !cpu_feature_enabled(X86_FEATURE_RETPOLINE_LFENCE))
return -1;

op = insn->opcode.bytes[0];
@@ -573,8 +575,9 @@ static int patch_retpoline(void *addr, s
* into:
*
* Jncc.d8 1f
+ * [ LFENCE ]
* JMP *%\reg
- * NOP
+ * [ NOP ]
* 1:
*/
/* Jcc.d32 second opcode byte is in the range: 0x80-0x8f */
@@ -589,6 +592,15 @@ static int patch_retpoline(void *addr, s
op = JMP32_INSN_OPCODE;
}

+ /*
+ * For RETPOLINE_AMD: prepend the indirect CALL/JMP with an LFENCE.
+ */
+ if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_LFENCE)) {
+ bytes[i++] = 0x0f;
+ bytes[i++] = 0xae;
+ bytes[i++] = 0xe8; /* LFENCE */
+ }
+
ret = emit_indirect(op, reg, bytes + i);
if (ret < 0)
return ret;