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Subject[tip: x86/urgent] x86/alternative: Optimize single-byte NOPs at an arbitrary position
The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: 2b31e8ed96b260ce2c22bd62ecbb9458399e3b62
Gitweb: https://git.kernel.org/tip/2b31e8ed96b260ce2c22bd62ecbb9458399e3b62
Author: Borislav Petkov <bp@suse.de>
AuthorDate: Tue, 01 Jun 2021 17:51:22 +02:00
Committer: Borislav Petkov <bp@suse.de>
CommitterDate: Thu, 03 Jun 2021 16:33:09 +02:00

x86/alternative: Optimize single-byte NOPs at an arbitrary position

Up until now the assumption was that an alternative patching site would
have some instructions at the beginning and trailing single-byte NOPs
(0x90) padding. Therefore, the patching machinery would go and optimize
those single-byte NOPs into longer ones.

However, this assumption is broken on 32-bit when code like
hv_do_hypercall() in hyperv_init() would use the ratpoline speculation
killer CALL_NOSPEC. The 32-bit version of that macro would align certain
insns to 16 bytes, leading to the compiler issuing a one or more
single-byte NOPs, depending on the holes it needs to fill for alignment.

That would lead to the warning in optimize_nops() to fire:

------------[ cut here ]------------
Not a NOP at 0xc27fb598
WARNING: CPU: 0 PID: 0 at arch/x86/kernel/alternative.c:211 optimize_nops.isra.13

due to that function verifying whether all of the following bytes really
are single-byte NOPs.

Therefore, carve out the NOP padding into a separate function and call
it for each NOP range beginning with a single-byte NOP.

Fixes: 23c1ad538f4f ("x86/alternatives: Optimize optimize_nops()")
Reported-by: Richard Narron <richard@aaazen.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=213301
Link: https://lkml.kernel.org/r/20210601212125.17145-1-bp@alien8.de
---
arch/x86/kernel/alternative.c | 64 ++++++++++++++++++++++++----------
1 file changed, 46 insertions(+), 18 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 6974b51..6fe5b44 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -183,41 +183,69 @@ done:
}

/*
+ * optimize_nops_range() - Optimize a sequence of single byte NOPs (0x90)
+ *
+ * @instr: instruction byte stream
+ * @instrlen: length of the above
+ * @off: offset within @instr where the first NOP has been detected
+ *
+ * Return: number of NOPs found (and replaced).
+ */
+static __always_inline int optimize_nops_range(u8 *instr, u8 instrlen, int off)
+{
+ unsigned long flags;
+ int i = off, nnops;
+
+ while (i < instrlen) {
+ if (instr[i] != 0x90)
+ break;
+
+ i++;
+ }
+
+ nnops = i - off;
+
+ if (nnops <= 1)
+ return nnops;
+
+ local_irq_save(flags);
+ add_nops(instr + off, nnops);
+ local_irq_restore(flags);
+
+ DUMP_BYTES(instr, instrlen, "%px: [%d:%d) optimized NOPs: ", instr, off, i);
+
+ return nnops;
+}
+
+/*
* "noinline" to cause control flow change and thus invalidate I$ and
* cause refetch after modification.
*/
static void __init_or_module noinline optimize_nops(struct alt_instr *a, u8 *instr)
{
- unsigned long flags;
struct insn insn;
- int nop, i = 0;
+ int i = 0;

/*
- * Jump over the non-NOP insns, the remaining bytes must be single-byte
- * NOPs, optimize them.
+ * Jump over the non-NOP insns and optimize single-byte NOPs into bigger
+ * ones.
*/
for (;;) {
if (insn_decode_kernel(&insn, &instr[i]))
return;

+ /*
+ * See if this and any potentially following NOPs can be
+ * optimized.
+ */
if (insn.length == 1 && insn.opcode.bytes[0] == 0x90)
- break;
-
- if ((i += insn.length) >= a->instrlen)
- return;
- }
+ i += optimize_nops_range(instr, a->instrlen, i);
+ else
+ i += insn.length;

- for (nop = i; i < a->instrlen; i++) {
- if (WARN_ONCE(instr[i] != 0x90, "Not a NOP at 0x%px\n", &instr[i]))
+ if (i >= a->instrlen)
return;
}
-
- local_irq_save(flags);
- add_nops(instr + nop, i - nop);
- local_irq_restore(flags);
-
- DUMP_BYTES(instr, a->instrlen, "%px: [%d:%d) optimized NOPs: ",
- instr, nop, a->instrlen);
}

/*
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