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Subject[tip: x86/core] x86/alternative: Relax text_poke_bp() constraint
The following commit has been merged into the x86/core branch of tip:

Commit-ID: 26c44b776dba4ac692a0bf5a3836feb8a63fea6b
Gitweb: https://git.kernel.org/tip/26c44b776dba4ac692a0bf5a3836feb8a63fea6b
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Sat, 04 Dec 2021 14:43:43 +01:00
Committer: Borislav Petkov <bp@suse.de>
CommitterDate: Thu, 09 Dec 2021 11:04:50 +01:00

x86/alternative: Relax text_poke_bp() constraint

Currently, text_poke_bp() is very strict to only allow patching a
single instruction; however with straight-line-speculation it will be
required to patch: ret; int3, which is two instructions.

As such, relax the constraints a little to allow int3 padding for all
instructions that do not imply the execution of the next instruction,
ie: RET, JMP.d8 and JMP.d32.

While there, rename the text_poke_loc::rel32 field to ::disp.

Note: this fills up the text_poke_loc structure which is now a round
16 bytes big.

[ bp: Put comments ontop instead of on the side. ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20211204134908.082342723@infradead.org
---
arch/x86/kernel/alternative.c | 49 +++++++++++++++++++++++-----------
1 file changed, 34 insertions(+), 15 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 175cde6..5007c3f 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -1113,10 +1113,13 @@ void text_poke_sync(void)
}

struct text_poke_loc {
- s32 rel_addr; /* addr := _stext + rel_addr */
- s32 rel32;
+ /* addr := _stext + rel_addr */
+ s32 rel_addr;
+ s32 disp;
+ u8 len;
u8 opcode;
const u8 text[POKE_MAX_OPCODE_SIZE];
+ /* see text_poke_bp_batch() */
u8 old;
};

@@ -1131,7 +1134,8 @@ static struct bp_patching_desc *bp_desc;
static __always_inline
struct bp_patching_desc *try_get_desc(struct bp_patching_desc **descp)
{
- struct bp_patching_desc *desc = __READ_ONCE(*descp); /* rcu_dereference */
+ /* rcu_dereference */
+ struct bp_patching_desc *desc = __READ_ONCE(*descp);

if (!desc || !arch_atomic_inc_not_zero(&desc->refs))
return NULL;
@@ -1165,7 +1169,7 @@ noinstr int poke_int3_handler(struct pt_regs *regs)
{
struct bp_patching_desc *desc;
struct text_poke_loc *tp;
- int len, ret = 0;
+ int ret = 0;
void *ip;

if (user_mode(regs))
@@ -1205,8 +1209,7 @@ noinstr int poke_int3_handler(struct pt_regs *regs)
goto out_put;
}

- len = text_opcode_size(tp->opcode);
- ip += len;
+ ip += tp->len;

switch (tp->opcode) {
case INT3_INSN_OPCODE:
@@ -1221,12 +1224,12 @@ noinstr int poke_int3_handler(struct pt_regs *regs)
break;

case CALL_INSN_OPCODE:
- int3_emulate_call(regs, (long)ip + tp->rel32);
+ int3_emulate_call(regs, (long)ip + tp->disp);
break;

case JMP32_INSN_OPCODE:
case JMP8_INSN_OPCODE:
- int3_emulate_jmp(regs, (long)ip + tp->rel32);
+ int3_emulate_jmp(regs, (long)ip + tp->disp);
break;

default:
@@ -1301,7 +1304,7 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
*/
for (do_sync = 0, i = 0; i < nr_entries; i++) {
u8 old[POKE_MAX_OPCODE_SIZE] = { tp[i].old, };
- int len = text_opcode_size(tp[i].opcode);
+ int len = tp[i].len;

if (len - INT3_INSN_SIZE > 0) {
memcpy(old + INT3_INSN_SIZE,
@@ -1378,21 +1381,37 @@ static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,
const void *opcode, size_t len, const void *emulate)
{
struct insn insn;
- int ret;
+ int ret, i;

memcpy((void *)tp->text, opcode, len);
if (!emulate)
emulate = opcode;

ret = insn_decode_kernel(&insn, emulate);
-
BUG_ON(ret < 0);
- BUG_ON(len != insn.length);

tp->rel_addr = addr - (void *)_stext;
+ tp->len = len;
tp->opcode = insn.opcode.bytes[0];

switch (tp->opcode) {
+ case RET_INSN_OPCODE:
+ case JMP32_INSN_OPCODE:
+ case JMP8_INSN_OPCODE:
+ /*
+ * Control flow instructions without implied execution of the
+ * next instruction can be padded with INT3.
+ */
+ for (i = insn.length; i < len; i++)
+ BUG_ON(tp->text[i] != INT3_INSN_OPCODE);
+ break;
+
+ default:
+ BUG_ON(len != insn.length);
+ };
+
+
+ switch (tp->opcode) {
case INT3_INSN_OPCODE:
case RET_INSN_OPCODE:
break;
@@ -1400,7 +1419,7 @@ static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,
case CALL_INSN_OPCODE:
case JMP32_INSN_OPCODE:
case JMP8_INSN_OPCODE:
- tp->rel32 = insn.immediate.value;
+ tp->disp = insn.immediate.value;
break;

default: /* assume NOP */
@@ -1408,13 +1427,13 @@ static void text_poke_loc_init(struct text_poke_loc *tp, void *addr,
case 2: /* NOP2 -- emulate as JMP8+0 */
BUG_ON(memcmp(emulate, x86_nops[len], len));
tp->opcode = JMP8_INSN_OPCODE;
- tp->rel32 = 0;
+ tp->disp = 0;
break;

case 5: /* NOP5 -- emulate as JMP32+0 */
BUG_ON(memcmp(emulate, x86_nops[len], len));
tp->opcode = JMP32_INSN_OPCODE;
- tp->rel32 = 0;
+ tp->disp = 0;
break;

default: /* unknown instruction */
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