[PATCH 1/4] x86/alternatives: Use a temporary buffer when optimizing NOPs

Borislav Petkov
2024-01-30T11:00:01Z

From: "Borislav Petkov (AMD)" <bp@alien8.de>

Instead of optimizing NOPs inplace, 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@alien8.de>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
---
arch/x86/kernel/alternative.c | 78 +++++++++++++++++++----------------
1 file changed, 42 insertions(+), 36 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index cc130b57542a..d633eb59f2b6 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: 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 __init_or_module add_nop(u8 *instr, unsigned int len)
+static void __init_or_module 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 __init_or_module
-__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
@@ -336,7 +340,7 @@ bool need_reloc(unsigned long offset, u8 *src, size_t src_len)
}

static void __init_or_module noinline
-apply_relocation(u8 *buf, size_t len, u8 *dest, u8 *src, size_t src_len)
+apply_relocation(const u8 * const instr, u8 *buf, size_t len, u8 *src, size_t src_len)
{
int prev, target = 0;

@@ -348,7 +352,7 @@ 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, len, &insn, &next, &prev, &target))
continue;

switch (insn.opcode.bytes[0]) {
@@ -365,7 +369,7 @@ apply_relocation(u8 *buf, size_t len, u8 *dest, u8 *src, size_t src_len)
if (need_reloc(next + insn.immediate.value, src, src_len)) {
apply_reloc(insn.immediate.nbytes,
buf + i + insn_offset_immediate(&insn),
- src - dest);
+ src - instr);
}

/*
@@ -373,7 +377,7 @@ 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 += src - instr;
imm += JMP32_INSN_SIZE - JMP8_INSN_SIZE;
if ((imm >> 31) == (imm >> 7)) {
buf[i+0] = JMP8_INSN_OPCODE;
@@ -389,7 +393,7 @@ apply_relocation(u8 *buf, size_t len, u8 *dest, u8 *src, size_t src_len)
if (need_reloc(next + insn.displacement.value, src, src_len)) {
apply_reloc(insn.displacement.nbytes,
buf + i + insn_offset_displacement(&insn),
- src - dest);
+ src - instr);
}
}
}
@@ -505,7 +509,9 @@ 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;
}

@@ -527,7 +533,7 @@ 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(instr, insn_buff, 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);
@@ -762,7 +768,7 @@ 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);
--
2.43.0