[PATCH v2 1/4] x86/alternatives: Use a temporary buffer when optimizing NOPs
From: Borislav Petkov
Date: Sun Mar 31 2024 - 14:01:25 EST
From: "Borislav Petkov (AMD)" <bp@xxxxxxxxx>
Instead of optimizing NOPs in-place, use a temporary buffer like the
usual alternatives patching flow does. This obviates the need to grab
locks when patching, see
6778977590da ("x86/alternatives: Disable interrupts and sync when optimizing NOPs in place")
While at it, add nomenclature definitions clarifying and simplifying the
naming of function-local variables in the alternatives code.
Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
---
arch/x86/include/asm/text-patching.h | 2 +-
arch/x86/kernel/alternative.c | 84 +++++++++++++++-------------
arch/x86/kernel/callthunks.c | 9 +--
3 files changed, 49 insertions(+), 46 deletions(-)
diff --git a/arch/x86/include/asm/text-patching.h b/arch/x86/include/asm/text-patching.h
index 345aafbc1964..6259f1937fe7 100644
--- a/arch/x86/include/asm/text-patching.h
+++ b/arch/x86/include/asm/text-patching.h
@@ -15,7 +15,7 @@
extern void text_poke_early(void *addr, const void *opcode, size_t len);
-extern void apply_relocation(u8 *buf, size_t len, u8 *dest, u8 *src, size_t src_len);
+extern void apply_relocation(u8 *buf, const u8 * const instr, size_t instrlen, u8 *repl, size_t repl_len);
/*
* Clear and restore the kernel write-protection flag on the local CPU.
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 45a280f2161c..ec94f1359c00 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -124,6 +124,20 @@ const unsigned char * const x86_nops[ASM_NOP_MAX+1] =
#endif
};
+/*
+ * Nomenclature for variable names to simplify and clarify this code and ease
+ * any potential staring at it:
+ *
+ * @instr: source address of the original instructions in the kernel text as
+ * generated by the compiler.
+ *
+ * @buf: temporary buffer on which the patching operates. This buffer is
+ * eventually text-poked into the kernel image.
+ *
+ * @replacement/@repl: pointer to the opcodes which are replacing @instr, located
+ * in the .altinstr_replacement section.
+ */
+
/*
* Fill the buffer with a single effective instruction of size @len.
*
@@ -133,28 +147,28 @@ const unsigned char * const x86_nops[ASM_NOP_MAX+1] =
* each single-byte NOPs). If @len to fill out is > ASM_NOP_MAX, pad with INT3 and
* *jump* over instead of executing long and daft NOPs.
*/
-static void add_nop(u8 *instr, unsigned int len)
+static void add_nop(u8 *buf, unsigned int len)
{
- u8 *target = instr + len;
+ u8 *target = buf + len;
if (!len)
return;
if (len <= ASM_NOP_MAX) {
- memcpy(instr, x86_nops[len], len);
+ memcpy(buf, x86_nops[len], len);
return;
}
if (len < 128) {
- __text_gen_insn(instr, JMP8_INSN_OPCODE, instr, target, JMP8_INSN_SIZE);
- instr += JMP8_INSN_SIZE;
+ __text_gen_insn(buf, JMP8_INSN_OPCODE, buf, target, JMP8_INSN_SIZE);
+ buf += JMP8_INSN_SIZE;
} else {
- __text_gen_insn(instr, JMP32_INSN_OPCODE, instr, target, JMP32_INSN_SIZE);
- instr += JMP32_INSN_SIZE;
+ __text_gen_insn(buf, JMP32_INSN_OPCODE, buf, target, JMP32_INSN_SIZE);
+ buf += JMP32_INSN_SIZE;
}
- for (;instr < target; instr++)
- *instr = INT3_INSN_OPCODE;
+ for (;buf < target; buf++)
+ *buf = INT3_INSN_OPCODE;
}
extern s32 __retpoline_sites[], __retpoline_sites_end[];
@@ -187,12 +201,12 @@ static bool insn_is_nop(struct insn *insn)
* Find the offset of the first non-NOP instruction starting at @offset
* but no further than @len.
*/
-static int skip_nops(u8 *instr, int offset, int len)
+static int skip_nops(u8 *buf, int offset, int len)
{
struct insn insn;
for (; offset < len; offset += insn.length) {
- if (insn_decode_kernel(&insn, &instr[offset]))
+ if (insn_decode_kernel(&insn, &buf[offset]))
break;
if (!insn_is_nop(&insn))
@@ -207,7 +221,7 @@ static int skip_nops(u8 *instr, int offset, int len)
* to the end of the NOP sequence into a single NOP.
*/
static bool
-__optimize_nops(u8 *instr, size_t len, struct insn *insn, int *next, int *prev, int *target)
+__optimize_nops(const u8 * const instr, u8 *buf, size_t len, struct insn *insn, int *next, int *prev, int *target)
{
int i = *next - insn->length;
@@ -222,12 +236,12 @@ __optimize_nops(u8 *instr, size_t len, struct insn *insn, int *next, int *prev,
if (insn_is_nop(insn)) {
int nop = i;
- *next = skip_nops(instr, *next, len);
+ *next = skip_nops(buf, *next, len);
if (*target && *next == *target)
nop = *prev;
- add_nop(instr + nop, *next - nop);
- DUMP_BYTES(ALT, instr, len, "%px: [%d:%d) optimized NOPs: ", instr, nop, *next);
+ add_nop(buf + nop, *next - nop);
+ DUMP_BYTES(ALT, buf, len, "%px: [%d:%d) optimized NOPs: ", instr, nop, *next);
return true;
}
@@ -239,32 +253,22 @@ __optimize_nops(u8 *instr, size_t len, struct insn *insn, int *next, int *prev,
* "noinline" to cause control flow change and thus invalidate I$ and
* cause refetch after modification.
*/
-static void __init_or_module noinline optimize_nops(u8 *instr, size_t len)
+static void __init_or_module noinline optimize_nops(const u8 * const instr, u8 *buf, size_t len)
{
int prev, target = 0;
for (int next, i = 0; i < len; i = next) {
struct insn insn;
- if (insn_decode_kernel(&insn, &instr[i]))
+ if (insn_decode_kernel(&insn, &buf[i]))
return;
next = i + insn.length;
- __optimize_nops(instr, len, &insn, &next, &prev, &target);
+ __optimize_nops(instr, buf, len, &insn, &next, &prev, &target);
}
}
-static void __init_or_module noinline optimize_nops_inplace(u8 *instr, size_t len)
-{
- unsigned long flags;
-
- local_irq_save(flags);
- optimize_nops(instr, len);
- sync_core();
- local_irq_restore(flags);
-}
-
/*
* In this context, "source" is where the instructions are placed in the
* section .altinstr_replacement, for example during kernel build by the
@@ -335,11 +339,11 @@ bool need_reloc(unsigned long offset, u8 *src, size_t src_len)
return (target < src || target > src + src_len);
}
-void apply_relocation(u8 *buf, size_t len, u8 *dest, u8 *src, size_t src_len)
+void apply_relocation(u8 *buf, const u8 * const instr, size_t instrlen, u8 *repl, size_t repl_len)
{
int prev, target = 0;
- for (int next, i = 0; i < len; i = next) {
+ for (int next, i = 0; i < instrlen; i = next) {
struct insn insn;
if (WARN_ON_ONCE(insn_decode_kernel(&insn, &buf[i])))
@@ -347,7 +351,7 @@ void apply_relocation(u8 *buf, size_t len, u8 *dest, u8 *src, size_t src_len)
next = i + insn.length;
- if (__optimize_nops(buf, len, &insn, &next, &prev, &target))
+ if (__optimize_nops(instr, buf, instrlen, &insn, &next, &prev, &target))
continue;
switch (insn.opcode.bytes[0]) {
@@ -361,10 +365,10 @@ void apply_relocation(u8 *buf, size_t len, u8 *dest, u8 *src, size_t src_len)
case JMP8_INSN_OPCODE:
case JMP32_INSN_OPCODE:
case CALL_INSN_OPCODE:
- if (need_reloc(next + insn.immediate.value, src, src_len)) {
+ if (need_reloc(next + insn.immediate.value, repl, repl_len)) {
apply_reloc(insn.immediate.nbytes,
buf + i + insn_offset_immediate(&insn),
- src - dest);
+ repl - instr);
}
/*
@@ -372,7 +376,7 @@ void apply_relocation(u8 *buf, size_t len, u8 *dest, u8 *src, size_t src_len)
*/
if (insn.opcode.bytes[0] == JMP32_INSN_OPCODE) {
s32 imm = insn.immediate.value;
- imm += src - dest;
+ imm += repl - instr;
imm += JMP32_INSN_SIZE - JMP8_INSN_SIZE;
if ((imm >> 31) == (imm >> 7)) {
buf[i+0] = JMP8_INSN_OPCODE;
@@ -385,10 +389,10 @@ void apply_relocation(u8 *buf, size_t len, u8 *dest, u8 *src, size_t src_len)
}
if (insn_rip_relative(&insn)) {
- if (need_reloc(next + insn.displacement.value, src, src_len)) {
+ if (need_reloc(next + insn.displacement.value, repl, repl_len)) {
apply_reloc(insn.displacement.nbytes,
buf + i + insn_offset_displacement(&insn),
- src - dest);
+ repl - instr);
}
}
}
@@ -504,7 +508,9 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
* patch if feature is *NOT* present.
*/
if (!boot_cpu_has(a->cpuid) == !(a->flags & ALT_FLAG_NOT)) {
- optimize_nops_inplace(instr, a->instrlen);
+ memcpy(insn_buff, instr, a->instrlen);
+ optimize_nops(instr, insn_buff, a->instrlen);
+ text_poke_early(instr, insn_buff, a->instrlen);
continue;
}
@@ -526,7 +532,7 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
for (; insn_buff_sz < a->instrlen; insn_buff_sz++)
insn_buff[insn_buff_sz] = 0x90;
- apply_relocation(insn_buff, a->instrlen, instr, replacement, a->replacementlen);
+ apply_relocation(insn_buff, instr, a->instrlen, replacement, a->replacementlen);
DUMP_BYTES(ALT, instr, a->instrlen, "%px: old_insn: ", instr);
DUMP_BYTES(ALT, replacement, a->replacementlen, "%px: rpl_insn: ", replacement);
@@ -761,7 +767,7 @@ void __init_or_module noinline apply_retpolines(s32 *start, s32 *end)
len = patch_retpoline(addr, &insn, bytes);
if (len == insn.length) {
- optimize_nops(bytes, len);
+ optimize_nops(addr, bytes, len);
DUMP_BYTES(RETPOLINE, ((u8*)addr), len, "%px: orig: ", addr);
DUMP_BYTES(RETPOLINE, ((u8*)bytes), len, "%px: repl: ", addr);
text_poke_early(addr, bytes, len);
diff --git a/arch/x86/kernel/callthunks.c b/arch/x86/kernel/callthunks.c
index 30335182b6b0..771d95484453 100644
--- a/arch/x86/kernel/callthunks.c
+++ b/arch/x86/kernel/callthunks.c
@@ -185,8 +185,7 @@ static void *patch_dest(void *dest, bool direct)
u8 *pad = dest - tsize;
memcpy(insn_buff, skl_call_thunk_template, tsize);
- apply_relocation(insn_buff, tsize, pad,
- skl_call_thunk_template, tsize);
+ apply_relocation(insn_buff, pad, tsize, skl_call_thunk_template, tsize);
/* Already patched? */
if (!bcmp(pad, insn_buff, tsize))
@@ -308,8 +307,7 @@ static bool is_callthunk(void *addr)
pad = (void *)(dest - tmpl_size);
memcpy(insn_buff, skl_call_thunk_template, tmpl_size);
- apply_relocation(insn_buff, tmpl_size, pad,
- skl_call_thunk_template, tmpl_size);
+ apply_relocation(insn_buff, pad, tmpl_size, skl_call_thunk_template, tmpl_size);
return !bcmp(pad, insn_buff, tmpl_size);
}
@@ -327,8 +325,7 @@ int x86_call_depth_emit_accounting(u8 **pprog, void *func)
return 0;
memcpy(insn_buff, skl_call_thunk_template, tmpl_size);
- apply_relocation(insn_buff, tmpl_size, *pprog,
- skl_call_thunk_template, tmpl_size);
+ apply_relocation(insn_buff, *pprog, tmpl_size, skl_call_thunk_template, tmpl_size);
memcpy(*pprog, insn_buff, tmpl_size);
*pprog += tmpl_size;
--
2.43.0