[patch 2/8] x86/fpu: Prevent state corruption in __fpu__restore_sig()

[Thomas Gleixner]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

The non-compacted slowpath uses __copy_from_user() and copies the entire
user buffer into the kernel buffer, verbatim.  This means that the kernel
buffer may now contain entirely invalid state on which XRSTOR will #GP.
validate_user_xstate_header() can detect some of that corruption, but that
leaves the onus on callers to clear the buffer.

Prior to XSAVES support it was possible just to reinitialize the buffer,
completely, but with supervisor states that is not longer possible as the
buffer clearing code split got it backwards. Fixing that is possible, but
not corrupting the state in the first place is more robust.

Avoid corruption of the kernel XSAVE buffer by using copy_user_to_xstate()
which validates the XSAVE header contents before copying the actual states
to the kernel. copy_user_to_xstate() was previously only called for
compacted-format kernel buffers, but it works for both compacted and
non-compacted forms.

Using it for the non-compacted form is slower because of multiple
__copy_from_user() operations, but that cost is less important than robust
code in an already slow path.

[ Changelog polished by Dave Hansen ]

Fixes: b860eb8dce59 ("x86/fpu/xstate: Define new functions for clearing fpregs and xstates")
Reported-by: syzbot+2067e764dbcd10721e2e@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 arch/x86/include/asm/fpu/xstate.h |    4 ----
 arch/x86/kernel/fpu/signal.c      |    9 +--------
 arch/x86/kernel/fpu/xstate.c      |    2 +-
 3 files changed, 2 insertions(+), 13 deletions(-)

--- a/arch/x86/include/asm/fpu/xstate.h
+++ b/arch/x86/include/asm/fpu/xstate.h
@@ -112,8 +112,4 @@ void copy_supervisor_to_kernel(struct xr
 void copy_dynamic_supervisor_to_kernel(struct xregs_state *xstate, u64 mask);
 void copy_kernel_to_dynamic_supervisor(struct xregs_state *xstate, u64 mask);
 
-
-/* Validate an xstate header supplied by userspace (ptrace or sigreturn) */
-int validate_user_xstate_header(const struct xstate_header *hdr);
-
 #endif
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -405,14 +405,7 @@ static int __fpu__restore_sig(void __use
 	if (use_xsave() && !fx_only) {
 		u64 init_bv = xfeatures_mask_user() & ~user_xfeatures;
 
-		if (using_compacted_format()) {
-			ret = copy_user_to_xstate(&fpu->state.xsave, buf_fx);
-		} else {
-			ret = __copy_from_user(&fpu->state.xsave, buf_fx, state_size);
-
-			if (!ret && state_size > offsetof(struct xregs_state, header))
-				ret = validate_user_xstate_header(&fpu->state.xsave.header);
-		}
+		ret = copy_user_to_xstate(&fpu->state.xsave, buf_fx);
 		if (ret)
 			goto err_out;
 
--- a/arch/x86/kernel/fpu/xstate.c
+++ b/arch/x86/kernel/fpu/xstate.c
@@ -515,7 +515,7 @@ int using_compacted_format(void)
 }
 
 /* Validate an xstate header supplied by userspace (ptrace or sigreturn) */
-int validate_user_xstate_header(const struct xstate_header *hdr)
+static int validate_user_xstate_header(const struct xstate_header *hdr)
 {
 	/* No unknown or supervisor features may be set */
 	if (hdr->xfeatures & ~xfeatures_mask_user())