On Wed, Jun 02, 2021 at 11:55:45AM +0200, Thomas Gleixner wrote:
> 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 ]
Nice polishing!
> 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)
Can't do that yet - that one is still called from regset.c:
arch/x86/kernel/fpu/regset.c: In function ‘xstateregs_set’:
arch/x86/kernel/fpu/regset.c:135:10: error: implicit declaration of function ‘validate_user_xstate_header’ [-Werror=implicit-function-declaration]
135 | ret = validate_user_xstate_header(&xsave->header);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: some warnings being treated as errors
Maybe after the 5th patch which kills that usage too.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
