Re: [patch V2 09/52] x86/fpu: Reject invalid MXCSR values in copy_kernel_to_xstate()

Borislav Petkov
2021-06-16T15:03:00Z

On Mon, Jun 14, 2021 at 05:44:17PM +0200, Thomas Gleixner wrote:
> Instead of masking out reserved bits, check them and reject the provided
> state as invalid if not zero.
>
> Suggested-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> V2: New patch
> ---
> arch/x86/kernel/fpu/xstate.c | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -1166,6 +1166,14 @@ int copy_kernel_to_xstate(struct xregs_s
> if (validate_user_xstate_header(&hdr))
> return -EINVAL;
>
> + if (xfeatures_mxcsr_quirk(hdr.xfeatures)) {

Since we're cleaning up this FPU stinking pile - that function needs to
have a verb in the name, something like:

if (xfeatures_mxcsr_quirk_needed(...))

but that's unrelated to here and as a note to whoever gets to get to it
first.

> + const u32 *mxcsr = kbuf + offsetof(struct fxregs_state, mxcsr);
> +
> + /* Reserved bits in MXCSR must be zero. */
> + if (*mxcsr & ~mxcsr_feature_mask)
> + return -EINVAL;
> + }

Btw, that function has another

if (xfeatures_mxcsr_quirk(hdr.xfeatures)) {

branch already below the loop.

Should we merge both? Diff ontop of yours:

---
diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 5d032c48f39d..30022d3fcd4a 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1172,6 +1172,11 @@ int copy_kernel_to_xstate(struct xregs_state *xsave, const void *kbuf)
/* Reserved bits in MXCSR must be zero. */
if (*mxcsr & ~mxcsr_feature_mask)
return -EINVAL;
+
+ offset = offsetof(struct fxregs_state, mxcsr);
+ size = MXCSR_AND_FLAGS_SIZE;
+
+ memcpy(&xsave->i387.mxcsr, kbuf + offset, size);
}

for (i = 0; i < XFEATURE_MAX; i++) {
@@ -1187,12 +1192,6 @@ int copy_kernel_to_xstate(struct xregs_state *xsave, const void *kbuf)
}
}

- if (xfeatures_mxcsr_quirk(hdr.xfeatures)) {
- offset = offsetof(struct fxregs_state, mxcsr);
- size = MXCSR_AND_FLAGS_SIZE;
- memcpy(&xsave->i387.mxcsr, kbuf + offset, size);
- }
-
/*
* The state that came in from userspace was user-state only.
* Mask all the user states out of 'xfeatures':

--
Regards/Gruss,
Boris.

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