Messages in this thread | | | Date | Wed, 2 Jun 2021 15:12:39 +0200 | From | Borislav Petkov <> | Subject | Re: [patch 2/8] x86/fpu: Prevent state corruption in __fpu__restore_sig() |
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On Wed, Jun 02, 2021 at 11:55:45AM +0200, Thomas Gleixner wrote: > From: Thomas Gleixner <tglx@linutronix.de> > > 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@syzkaller.appspotmail.com > Signed-off-by: Thomas Gleixner <tglx@linutronix.de> > Cc: stable@vger.kernel.org > --- > 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
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