lkml.org 
[lkml]   [2018]   [Nov]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    From
    Subject[tip:x86/mm] x86/fault: Fix SMAP #PF handling buglet for implicit supervisor accesses
    Commit-ID:  e50928d7213e72ee95507221a89ed07d2bb6517b
    Gitweb: https://git.kernel.org/tip/e50928d7213e72ee95507221a89ed07d2bb6517b
    Author: Andy Lutomirski <luto@kernel.org>
    AuthorDate: Mon, 19 Nov 2018 14:45:29 -0800
    Committer: Ingo Molnar <mingo@kernel.org>
    CommitDate: Tue, 20 Nov 2018 08:44:29 +0100

    x86/fault: Fix SMAP #PF handling buglet for implicit supervisor accesses

    Currently, if a user program somehow triggers an implicit supervisor
    access to a user address (e.g. if the kernel somehow sets LDTR to a
    user address), it will be incorrectly detected as a SMAP violation
    if AC is clear and SMAP is enabled. This is incorrect -- the error
    has nothing to do with SMAP. Fix the condition so that only
    accesses with the hardware USER bit set are diagnosed as SMAP
    violations.

    With the logic fixed, an implicit supervisor access to a user address
    will hit the code lower in the function that is intended to handle it
    even if SMAP is enabled. That logic is still a bit buggy, and later
    patches will clean it up.

    I *think* this code is still correct for WRUSS, and I've added a
    comment to that effect.

    Signed-off-by: Andy Lutomirski <luto@kernel.org>
    Cc: Andy Lutomirski <luto@amacapital.net>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Brian Gerst <brgerst@gmail.com>
    Cc: Dave Hansen <dave.hansen@linux.intel.com>
    Cc: Denys Vlasenko <dvlasenk@redhat.com>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Rik van Riel <riel@surriel.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Yu-cheng Yu <yu-cheng.yu@intel.com>
    Link: http://lkml.kernel.org/r/d1d1b2e66ef31f884dba172084486ea9423ddcdb.1542667307.git.luto@kernel.org
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    ---
    arch/x86/mm/fault.c | 9 ++++++---
    1 file changed, 6 insertions(+), 3 deletions(-)

    diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
    index 9d092ab74f18..7a69b66cf071 100644
    --- a/arch/x86/mm/fault.c
    +++ b/arch/x86/mm/fault.c
    @@ -1235,12 +1235,15 @@ void do_user_addr_fault(struct pt_regs *regs,
    pgtable_bad(regs, hw_error_code, address);

    /*
    - * If SMAP is on, check for invalid kernel (supervisor)
    - * access to user pages in the user address space.
    + * If SMAP is on, check for invalid kernel (supervisor) access to user
    + * pages in the user address space. The odd case here is WRUSS,
    + * which, according to the preliminary documentation, does not respect
    + * SMAP and will have the USER bit set so, in all cases, SMAP
    + * enforcement appears to be consistent with the USER bit.
    */
    if (unlikely(cpu_feature_enabled(X86_FEATURE_SMAP) &&
    !(hw_error_code & X86_PF_USER) &&
    - (user_mode(regs) || !(regs->flags & X86_EFLAGS_AC))))
    + !(regs->flags & X86_EFLAGS_AC)))
    {
    bad_area_nosemaphore(regs, hw_error_code, address);
    return;
    \
     
     \ /
      Last update: 2018-11-20 09:17    [W:4.184 / U:0.280 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site