Re: [PATCH 0/2] wireguard-linux-compat: grsecurity compat patches

[Mathias Krause <minipli@grsecurity.net>]

Hi Jason,

Am 06.12.21 um 17:27 schrieb Jason A. Donenfeld:
> Oh, you're right about recent gcc. That actually _is_ intended, yet
> they still fail. It would seem, then, that the problem is not so much
> gcc version as it is some kernel patch that never made it to these
> ancient kernels.

actually, it's the i/o constraints, they're wrong. 'out' is an input
operand but we specify it as an output one. Now this works when gcc
respects the "+" constraint, as in marking this operand as being read
and written, thereby implicitly requiring it to be initialized. But
looks like older gcc ignore that (at least when using alternatives) and
make the asm work on a stale 'out' operand, resulting in the selftest
failures and crashes you've seen.

The following change fixes it by putting 'out' to the input operand
list, where it really belongs to:

diff --git a/src/crypto/zinc/curve25519/curve25519-x86_64.c
b/src/crypto/zinc/curve25519/curve25519-x86_64.c
index 67f55affcf88..f26ed5d897ac 100644
--- a/src/crypto/zinc/curve25519/curve25519-x86_64.c
+++ b/src/crypto/zinc/curve25519/curve25519-x86_64.c
@@ -581,8 +581,8 @@ static inline void fsqr(u64 *out, const u64 *f, u64
*tmp)
                "  cmovc %%rdx, %%rax;"
                "  add %%rax, %%r8;"
                "  movq %%r8, 0(%0);"
-       : "+&r,&r" (tmp), "+&r,&r" (f), "+&r,m" (out)
-       :
+       : "+&r,&r" (tmp), "+&r,&r" (f)
+       : "r,m" (out)
        : "%rax", "%rcx", "%rdx", "%r8", "%r9", "%r10", "%r11", "%rbx",
"%r13", "%r14", "%r15", "memory", "cc"
        );
 }
@@ -743,8 +743,8 @@ static inline void fsqr2(u64 *out, const u64 *f, u64
*tmp)
                "  cmovc %%rdx, %%rax;"
                "  add %%rax, %%r8;"
                "  movq %%r8, 32(%0);"
-       : "+&r,&r" (tmp), "+&r,&r" (f), "+&r,m" (out)
-       :
+       : "+&r,&r" (tmp), "+&r,&r" (f)
+       : "r,m" (out)
        : "%rax", "%rcx", "%rdx", "%r8", "%r9", "%r10", "%r11", "%rbx",
"%r13", "%r14", "%r15", "memory", "cc"
        );
 }

We still need the early clobber constraint ("&") for 'tmp' and 'f' as
they are, in fact, written to early. But 'out' is only ever read, so can
be a normal input operand.

I'll create a proper patch and send it out tomorrow, if you don't beat
me to.


Thanks,
Mathias