Messages in this thread | | | Date | Wed, 2 Dec 2020 11:07:26 -0800 | From | Kees Cook <> | Subject | Re: [PATCH 1/3] x86/sev-es: Fix not using prefixes.nbytes for loop over prefixes.bytes |
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On Wed, Dec 02, 2020 at 09:31:57AM -0600, Tom Lendacky wrote: > On 12/2/20 2:51 AM, Masami Hiramatsu wrote: > > Since the insn.prefixes.nbytes can be bigger than the size of > > insn.prefixes.bytes[] when a same prefix is repeated, we have to > > check whether the insn.prefixes.bytes[i] != 0 and i < 4 instead > > of insn.prefixes.nbytes. > > > > Fixes: 25189d08e516 ("x86/sev-es: Add support for handling IOIO exceptions") > > Reported-by: Kees Cook <keescook@chromium.org> > > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org> > > --- > > arch/x86/boot/compressed/sev-es.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/arch/x86/boot/compressed/sev-es.c b/arch/x86/boot/compressed/sev-es.c > > index 954cb2702e23..6a7a3027c9ac 100644 > > --- a/arch/x86/boot/compressed/sev-es.c > > +++ b/arch/x86/boot/compressed/sev-es.c > > @@ -36,7 +36,7 @@ static bool insn_has_rep_prefix(struct insn *insn) > > insn_get_prefixes(insn); > > - for (i = 0; i < insn->prefixes.nbytes; i++) { > > + for (i = 0; insn->prefixes.bytes[i] && i < 4; i++) {
You must test "i" before bytes[i] or you still do the out-of-bounds-read.
> > Wouldn't it be better to create a #define for the size rather than hard > coding 4 in the various files? That would protect everything should the > bytes array size ever change in the future.
Agreed, and perhaps instead of repeating the idiom in the for loop, add a helper like:
#define insn_prefix_valid(prefixes, i) (i >=0 && i < 4 && prefixes->bytes[i])
to be used like:
for (i = 0; insn_prefix_valid(&insn->prefixes, i); i++) {
> > Thanks, > Tom > > > insn_byte_t p = insn->prefixes.bytes[i]; > > if (p == 0xf2 || p == 0xf3) > >
-- Kees Cook
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