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Subject[tip: x86/fpu] x86/fpu: Reject invalid MXCSR values in copy_kernel_to_xstate()
The following commit has been merged into the x86/fpu branch of tip:

Commit-ID: 947f4947cf00ea1e6d319eb182c64ea51ba4de8d
Gitweb: https://git.kernel.org/tip/947f4947cf00ea1e6d319eb182c64ea51ba4de8d
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Wed, 23 Jun 2021 14:01:37 +02:00
Committer: Borislav Petkov <bp@suse.de>
CommitterDate: Wed, 23 Jun 2021 17:49:46 +02:00

x86/fpu: Reject invalid MXCSR values in copy_kernel_to_xstate()

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>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121452.308388343@linutronix.de
---
arch/x86/kernel/fpu/xstate.c | 19 ++++++++++++++++---
1 file changed, 16 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
index 2b7b579..9cf84c5 100644
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -1154,6 +1154,19 @@ void copy_xstate_to_kernel(struct membuf to, struct xregs_state *xsave)
membuf_zero(&to, to.left);
}

+static inline bool mxcsr_valid(struct xstate_header *hdr, const u32 *mxcsr)
+{
+ u64 mask = XFEATURE_MASK_FP | XFEATURE_MASK_SSE | XFEATURE_MASK_YMM;
+
+ /* Only check if it is in use */
+ if (hdr->xfeatures & mask) {
+ /* Reserved bits in MXCSR must be zero. */
+ if (*mxcsr & ~mxcsr_feature_mask)
+ return false;
+ }
+ return true;
+}
+
/*
* Convert from a ptrace standard-format kernel buffer to kernel XSAVE[S] format
* and copy to the target thread. This is called from xstateregs_set().
@@ -1172,6 +1185,9 @@ int copy_kernel_to_xstate(struct xregs_state *xsave, const void *kbuf)
if (validate_user_xstate_header(&hdr))
return -EINVAL;

+ if (!mxcsr_valid(&hdr, kbuf + offsetof(struct fxregs_state, mxcsr)))
+ return -EINVAL;
+
for (i = 0; i < XFEATURE_MAX; i++) {
u64 mask = ((u64)1 << i);

@@ -1202,9 +1218,6 @@ int copy_kernel_to_xstate(struct xregs_state *xsave, const void *kbuf)
*/
xsave->header.xfeatures |= hdr.xfeatures;

- /* mxcsr reserved bits must be masked to zero for historical reasons. */
- xsave->i387.mxcsr &= mxcsr_feature_mask;
-
return 0;
}

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