On Fri, Apr 11, 2025 at 06:32:43PM +0200, Oleg Nesterov wrote:
> On 04/11, Andrii Nakryiko wrote:
> >
> > > --- a/arch/x86/kernel/uprobes.c
> > > +++ b/arch/x86/kernel/uprobes.c
> > > @@ -840,6 +840,12 @@ static int branch_setup_xol_ops(struct arch_uprobe
> > > *auprobe, struct insn *insn)
> > > insn_byte_t p;
> > > int i;
> > >
> > > + /* x86_nops[i]; same as jmp with .offs = 0 */
> > > + for (i = 1; i <= ASM_NOP_MAX; ++i) {
> >
> > i <= ASM_NOP_MAX && i <= insn->length
> >
> > ?
> >
> > otherwise what prevents us from reading past the actual instruction bytes?
>
> Well, copy_insn() just copies MAX_UINSN_BYTES into arch_uprobe.insn[].
> If, say, the 1st 11 bytes of arch_uprobe.insn (or insn->kaddr) match
> x86_nops[11] then insn->length must be 11, or insn_decode() is buggy?
>
> > or, actually, shouldn't we just check memcmp(x86_nops[insn->length])
> > if insn->length < ASM_NOP_MAX ?
nice, did not think of that
>
> Hmm... agreed.
>
> Either way this check can't (doesn't even try to) detect, say,
> "rep; BYTES_NOP5", so we do not care if insn->length == 6 in this case.
>
> Good point!
I'll run tests and send formal patch for change below
thanks,
jirka
---
diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
index 9194695662b2..6d383839e839 100644
--- a/arch/x86/kernel/uprobes.c
+++ b/arch/x86/kernel/uprobes.c
@@ -840,6 +840,11 @@ static int branch_setup_xol_ops(struct arch_uprobe
*auprobe, struct insn *insn)
insn_byte_t p;
int i;
+ /* x86_nops[insn->length]; same as jmp with .offs = 0 */
+ if (insn->length <= ASM_NOP_MAX &&
+ !memcmp(insn->kaddr, x86_nops[insn->length], insn->length))
+ goto setup;
+
switch (opc1) {
case 0xeb: /* jmp 8 */
case 0xe9: /* jmp 32 */
