Upstream 814fb7bb7db54 ("x86/fpu: Don't let userspace set bogus xcomp_bv").

Guenter, do you plan to backport the fix to 4.4 stable?

#2: It is already in chromeos-4.4 (commit d25fea066a8e) through the 4.4.92 merge. Do you mean to R63 ?

Yeah, I meant did you plan to have it in 63 and 62 as well? I am not sure how severe it really is, but for lakitu we usually patch stable unless it's clearly not needed.

#4: Not really, given the low severity. I don't mind doing it, though. If you think it is necessary/warranted for Lakitu, please update the label and add a note describing the reasons.
Note that fixing his only really makes sense for chromeos-4.4 - older kernels leak kernel data all over the place, and fixing a single instance would be like a drop of water in an ocean.

Thanks. Lakitu only cares about 4.4.

Requesting merging all the way to 61 as lakitu still supports it.

Guenter, let me know if you need help with the additional cherrypicks.

Requesting merge to R61..R63 per #4, #6. Fix is already in ToT.

This bug requires manual review: M63 has already been promoted to the beta branch, so this requires manual review
Please contact the milestone owner if you have questions.
Owners: cmasso@(Android), cmasso@(iOS), gkihumba@(ChromeOS), govind@(Desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Guenter, I noticed that you removed Merge-Request-61, but lakitu would still like it in 61 so I added it back. Let me know if you have any concern though.

Sheriffbot removed M-61, as seen in #10. That suggested to me that M-61 is obsolete. Maybe not, but you might want take this up with the Sheriffbot maintainers. I don't really want to start an argument with an automaton.

Too late for M61 but approving M-62 and M-63 

Thanks. For lakitu we support two stable branches with the same priority so if anything is to be backported to 62, we would like it in 61 as well. For lakitu, 61 is more than a month away from eol, and there are a lot of users on it currently.

That said, upon further internal discussion, this CVE isn't severe enough to be backported to stable. So I'll remove the merge-approval for 62 and only backport it to 63 (currently beta). Sorry about the noise!

Assigning back to Guenter since he had prepared the cherrypick CL.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/third_party/kernel/+/e8f7ba87c48082bfe91ce13c580a7df102eded33

commit e8f7ba87c48082bfe91ce13c580a7df102eded33
Author: Eric Biggers <ebiggers@google.com>
Date: Mon Nov 06 23:17:33 2017

UPSTREAM: x86/fpu: Don't let userspace set bogus xcomp_bv

commit 814fb7bb7db5433757d76f4c4502c96fc53b0b5e upstream.

[Please apply to 4.4-stable.  Note: the backport includes the
fpstate_init() call in xstateregs_set(), since fix is useless without
it.  It was added by commit 91c3dba7dbc1 ("x86/fpu/xstate: Fix PTRACE
frames for XSAVES"), but it doesn't make sense to backport that whole
commit.]

On x86, userspace can use the ptrace() or rt_sigreturn() system calls to
set a task's extended state (xstate) or "FPU" registers.  ptrace() can
set them for another task using the PTRACE_SETREGSET request with
NT_X86_XSTATE, while rt_sigreturn() can set them for the current task.
In either case, registers can be set to any value, but the kernel
assumes that the XSAVE area itself remains valid in the sense that the
CPU can restore it.

However, in the case where the kernel is using the uncompacted xstate
format (which it does whenever the XSAVES instruction is unavailable),
it was possible for userspace to set the xcomp_bv field in the
xstate_header to an arbitrary value.  However, all bits in that field
are reserved in the uncompacted case, so when switching to a task with
nonzero xcomp_bv, the XRSTOR instruction failed with a #GP fault.  This
caused the WARN_ON_FPU(err) in copy_kernel_to_xregs() to be hit.  In
addition, since the error is otherwise ignored, the FPU registers from
the task previously executing on the CPU were leaked.

Fix the bug by checking that the user-supplied value of xcomp_bv is 0 in
the uncompacted case, and returning an error otherwise.

The reason for validating xcomp_bv rather than simply overwriting it
with 0 is that we want userspace to see an error if it (incorrectly)
provides an XSAVE area in compacted format rather than in uncompacted
format.

Note that as before, in case of error we clear the task's FPU state.
This is perhaps non-ideal, especially for PTRACE_SETREGSET; it might be
better to return an error before changing anything.  But it seems the
"clear on error" behavior is fine for now, and it's a little tricky to
do otherwise because it would mean we couldn't simply copy the full
userspace state into kernel memory in one __copy_from_user().

This bug was found by syzkaller, which hit the above-mentioned
WARN_ON_FPU():

    WARNING: CPU: 1 PID: 0 at ./arch/x86/include/asm/fpu/internal.h:373 __switch_to+0x5b5/0x5d0
    CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.13.0 #453
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
    task: ffff9ba2bc8e42c0 task.stack: ffffa78cc036c000
    RIP: 0010:__switch_to+0x5b5/0x5d0
    RSP: 0000:ffffa78cc08bbb88 EFLAGS: 00010082
    RAX: 00000000fffffffe RBX: ffff9ba2b8bf2180 RCX: 00000000c0000100
    RDX: 00000000ffffffff RSI: 000000005cb10700 RDI: ffff9ba2b8bf36c0
    RBP: ffffa78cc08bbbd0 R08: 00000000929fdf46 R09: 0000000000000001
    R10: 0000000000000000 R11: 0000000000000000 R12: ffff9ba2bc8e42c0
    R13: 0000000000000000 R14: ffff9ba2b8bf3680 R15: ffff9ba2bf5d7b40
    FS:  00007f7e5cb10700(0000) GS:ffff9ba2bf400000(0000) knlGS:0000000000000000
    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: 00000000004005cc CR3: 0000000079fd5000 CR4: 00000000001406e0
    Call Trace:
    Code: 84 00 00 00 00 00 e9 11 fd ff ff 0f ff 66 0f 1f 84 00 00 00 00 00 e9 e7 fa ff ff 0f ff 66 0f 1f 84 00 00 00 00 00 e9 c2 fa ff ff <0f> ff 66 0f 1f 84 00 00 00 00 00 e9 d4 fc ff ff 66 66 2e 0f 1f

Here is a C reproducer.  The expected behavior is that the program spin
forever with no output.  However, on a buggy kernel running on a
processor with the "xsave" feature but without the "xsaves" feature
(e.g. Sandy Bridge through Broadwell for Intel), within a second or two
the program reports that the xmm registers were corrupted, i.e. were not
restored correctly.  With CONFIG_X86_DEBUG_FPU=y it also hits the above
kernel warning.

    #define _GNU_SOURCE
    #include <stdbool.h>
    #include <inttypes.h>
    #include <linux/elf.h>
    #include <stdio.h>
    #include <sys/ptrace.h>
    #include <sys/uio.h>
    #include <sys/wait.h>
    #include <unistd.h>

    int main(void)
    {
        int pid = fork();
        uint64_t xstate[512];
        struct iovec iov = { .iov_base = xstate, .iov_len = sizeof(xstate) };

        if (pid == 0) {
            bool tracee = true;
            for (int i = 0; i < sysconf(_SC_NPROCESSORS_ONLN) && tracee; i++)
                tracee = (fork() != 0);
            uint32_t xmm0[4] = { [0 ... 3] = tracee ? 0x00000000 : 0xDEADBEEF };
            asm volatile("   movdqu %0, %%xmm0\n"
                         "   mov %0, %%rbx\n"
                         "1: movdqu %%xmm0, %0\n"
                         "   mov %0, %%rax\n"
                         "   cmp %%rax, %%rbx\n"
                         "   je 1b\n"
                         : "+m" (xmm0) : : "rax", "rbx", "xmm0");
            printf("BUG: xmm registers corrupted!  tracee=%d, xmm0=%08X%08X%08X%08X\n",
                   tracee, xmm0[0], xmm0[1], xmm0[2], xmm0[3]);
        } else {
            usleep(100000);
            ptrace(PTRACE_ATTACH, pid, 0, 0);
            wait(NULL);
            ptrace(PTRACE_GETREGSET, pid, NT_X86_XSTATE, &iov);
            xstate[65] = -1;
            ptrace(PTRACE_SETREGSET, pid, NT_X86_XSTATE, &iov);
            ptrace(PTRACE_CONT, pid, 0, 0);
            wait(NULL);
        }
        return 1;
    }

Note: the program only tests for the bug using the ptrace() system call.
The bug can also be reproduced using the rt_sigreturn() system call, but
only when called from a 32-bit program, since for 64-bit programs the
kernel restores the FPU state from the signal frame by doing XRSTOR
directly from userspace memory (with proper error checking).


BUG= chromium:780784 
TEST=Run C-Reproducer after applying the fix

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Rik van Riel <riel@redhat.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Eric Biggers <ebiggers3@gmail.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Kevin Hao <haokexin@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Halcrow <mhalcrow@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Wanpeng Li <wanpeng.li@hotmail.com>
Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: kernel-hardening@lists.openwall.com
Fixes: 0b29643a5843 ("x86/xsaves: Change compacted format xsave area header")
Link: http://lkml.kernel.org/r/20170922174156.16780-2-ebiggers3@gmail.com
Link: http://lkml.kernel.org/r/20170923130016.21448-25-mingo@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from chromeos-4.4,
 commit d25fea066a8ed4457a9f8b23eb78204b9b6896cf)
Signed-off-by: Guenter Roeck <groeck@chromium.org>

Change-Id: I6a28eff31c00a1f4209d9616788606b8a4100cb2
Reviewed-on: https://chromium-review.googlesource.com/755345
Reviewed-by: Guenter Roeck <groeck@chromium.org>
Reviewed-by: Daniel Wang <wonderfly@google.com>
Commit-Queue: Guenter Roeck <groeck@chromium.org>
Tested-by: Guenter Roeck <groeck@chromium.org>

[modify] https://crrev.com/e8f7ba87c48082bfe91ce13c580a7df102eded33/arch/x86/kernel/fpu/regset.c
[modify] https://crrev.com/e8f7ba87c48082bfe91ce13c580a7df102eded33/arch/x86/kernel/fpu/signal.c

This bug has been closed for more than 14 weeks. Removing security view restrictions.

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