Re: [PATCH v3 0/3] x86/fpu: prevent leaking FPU registers via invalid FPU state

Eric Biggers
2017-09-22T17:07:29Z

On Fri, Sep 22, 2017 at 07:33:14AM +0200, Ingo Molnar wrote:
>
> * Eric Biggers <ebiggers3@gmail.com> wrote:
>
> > From: Eric Biggers <ebiggers@google.com>
> >
> > This series fixes the bug found by syzkaller where the ptrace syscall
> > can be used to set invalid bits in a task's FPU state. I also found
> > that an equivalent bug was reachable using the sigreturn syscall, so the
> > first patch fixes the bug in both cases.
> >
> > The other two patches start validating the other parts of the
> > xstate_header and make it so that invalid FPU states can no longer be
> > abused to leak the FPU registers of other processes.
> >
> > Changes since v2:
> > - Use an exception handler to handle invalid FPU states
> > (suggested by Andy Lutomirski)
> > - Check the size of xstate_header.reserved at build time
> > (suggested by Dave Hansen)
> >
> > Eric Biggers (3):
> > x86/fpu: don't let userspace set bogus xcomp_bv
> > x86/fpu: tighten validation of user-supplied xstate_header
> > x86/fpu: reinitialize FPU registers if restoring FPU state fails
> >
> > arch/x86/include/asm/fpu/internal.h | 51 +++++++++++--------------------------
> > arch/x86/include/asm/fpu/xstate.h | 25 ++++++++++++++++++
> > arch/x86/kernel/fpu/regset.c | 20 +++++++--------
> > arch/x86/kernel/fpu/signal.c | 15 ++++++++---
> > arch/x86/kernel/fpu/xstate.c | 27 ++++++++------------
> > arch/x86/mm/extable.c | 24 +++++++++++++++++
> > 6 files changed, 94 insertions(+), 68 deletions(-)
>
> Ok - could you please rebase these to to tip:master that is at:
>
> git git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git master
>
> In particular this has a WIP.x86/fpu branch with FPU fixes+changes queued up but
> not merged upstream (yet), which conflict with these changes. I'd like to merge
> them all together.
>

Working on it, but there is a problem with current tip. PTRACE_GETREGSET is
causing the following warning:

[ 5.024462] WARNING: CPU: 1 PID: 347 at arch/x86/kernel/fpu/core.c:147 fpu__save+0x2ab/0x2c0
[ 5.025030] CPU: 1 PID: 347 Comm: reproduce Not tainted 4.14.0-rc1-00274-ga4cc07ed56ab #615
[ 5.025639] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
[ 5.026183] task: ffff8af778664380 task.stack: ffff98f6008f8000
[ 5.026590] RIP: 0010:fpu__save+0x2ab/0x2c0
[ 5.027376] RSP: 0018:ffff98f6008fbd90 EFLAGS: 00010202
[ 5.027751] RAX: ffff8af778665a40 RBX: ffff8af779ccfb80 RCX: 0000000000000340
[ 5.028235] RDX: 0000000000000000 RSI: ffffffffb566d348 RDI: ffff8af779ccfb80
[ 5.028718] RBP: ffff98f6008fbda0 R08: 0000000000000000 R09: 00007fffb907e870
[ 5.029205] R10: ffffffffb4a25780 R11: ffffffffb5404c60 R12: ffff8af779ccfb80
[ 5.029685] R13: 0000000000000340 R14: ffff8af779cce4c0 R15: ffff8af779ccfb80
[ 5.030557] FS: 00007f662220d700(0000) GS:ffff8af77f400000(0000) knlGS:0000000000000000
[ 5.031114] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 5.031504] CR2: 00007fa712274008 CR3: 0000000078730003 CR4: 00000000001606e0
[ 5.031990] Call Trace:
[ 5.032166] fpu__activate_fpstate_read+0x90/0x240
[ 5.032493] xstateregs_get+0x53/0x160
[ 5.032767] ptrace_regset+0x123/0x180
[ 5.033034] ptrace_request+0x460/0x590
[ 5.033300] ? wait_task_inactive+0xdb/0x270
[ 5.043352] arch_ptrace+0x34c/0x3d0
[ 5.043631] ? ptrace_check_attach+0xd1/0x120
[ 5.043929] SyS_ptrace+0xa6/0x100
[ 5.044164] entry_SYSCALL_64_fastpath+0x1f/0xbe
[ 5.044478] RIP: 0033:0x7f6621b17c6e
[ 5.044724] RSP: 002b:00007fffb907e838 EFLAGS: 00000202 ORIG_RAX: 0000000000000065
[ 5.045233] RAX: ffffffffffffffda RBX: 0000000000601050 RCX: 00007f6621b17c6e
[ 5.045711] RDX: 0000000000000202 RSI: 000000000000015c RDI: 0000000000004204
[ 5.046189] RBP: 00000000004008e0 R08: 0000000000004203 R09: 000000000000015b
[ 5.046668] R10: 00007fffb907e850 R11: 0000000000000202 R12: 00000000004007f2
[ 5.047777] R13: 00007fffb907f960 R14: 0000000000000000 R15: 0000000000000000
[ 5.048262] Code: c0 0f 85 ce fe ff ff 48 c7 c2 88 34 5c b5 be 2c 00 00 00 48 c7 c7 e0 5f 5c b5 c6 05 49 71 f0 00 01 e8 da ae 0a 00 e9 aa fe ff ff <0f> ff e9 75 fd ff ff 0f ff e9 b5 fd ff ff 0f 1f 80 00 00 00 00