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    Subject[PATCH] x86/asm/entry/64: better check for canonical address
    Date
    This change makes the check exact (no more false positives
    on kernel addresses).

    It isn't really important to be fully correct here -
    almost all addresses we'll ever see will be userspace ones,
    but OTOH it looks to be cheap enough:
    the new code uses two more ALU ops but preserves %rcx,
    allowing to not reload it from pt_regs->cx again.
    On disassembly level, the changes are:

    cmp %rcx,0x80(%rsp) -> mov 0x80(%rsp),%r11; cmp %rcx,%r11
    shr $0x2f,%rcx -> shl $0x10,%rcx; sar $0x10,%rcx; cmp %rcx,%r11
    mov 0x58(%rsp),%rcx -> (eliminated)

    Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
    CC: Borislav Petkov <bp@alien8.de>
    CC: x86@kernel.org
    CC: linux-kernel@vger.kernel.org
    ---

    Andy, I'd undecided myself on the merits of doing this.
    If you like it, feel free to take it in your tree.
    I trimmed CC list to not bother too many people with this trivial
    and quite possibly "useless churn"-class change.

    arch/x86/kernel/entry_64.S | 23 ++++++++++++-----------
    1 file changed, 12 insertions(+), 11 deletions(-)

    diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
    index bf9afad..a36d04d 100644
    --- a/arch/x86/kernel/entry_64.S
    +++ b/arch/x86/kernel/entry_64.S
    @@ -688,26 +688,27 @@ retint_swapgs: /* return to user-space */
    * a completely clean 64-bit userspace context.
    */
    movq RCX(%rsp),%rcx
    - cmpq %rcx,RIP(%rsp) /* RCX == RIP */
    + movq RIP(%rsp),%r11
    + cmpq %rcx,%r11 /* RCX == RIP */
    jne opportunistic_sysret_failed

    /*
    * On Intel CPUs, sysret with non-canonical RCX/RIP will #GP
    * in kernel space. This essentially lets the user take over
    - * the kernel, since userspace controls RSP. It's not worth
    - * testing for canonicalness exactly -- this check detects any
    - * of the 17 high bits set, which is true for non-canonical
    - * or kernel addresses. (This will pessimize vsyscall=native.
    - * Big deal.)
    + * the kernel, since userspace controls RSP.
    *
    - * If virtual addresses ever become wider, this will need
    + * If width of "canonical tail" ever become variable, this will need
    * to be updated to remain correct on both old and new CPUs.
    */
    .ifne __VIRTUAL_MASK_SHIFT - 47
    .error "virtual address width changed -- sysret checks need update"
    .endif
    - shr $__VIRTUAL_MASK_SHIFT, %rcx
    - jnz opportunistic_sysret_failed
    + /* Change top 16 bits to be a sign-extension of the rest */
    + shl $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
    + sar $(64 - (__VIRTUAL_MASK_SHIFT+1)), %rcx
    + /* If this changed %rcx, it was not canonical */
    + cmpq %rcx, %r11
    + jne opportunistic_sysret_failed

    cmpq $__USER_CS,CS(%rsp) /* CS must match SYSRET */
    jne opportunistic_sysret_failed
    @@ -730,8 +731,8 @@ retint_swapgs: /* return to user-space */
    */
    irq_return_via_sysret:
    CFI_REMEMBER_STATE
    - /* r11 is already restored (see code above) */
    - RESTORE_C_REGS_EXCEPT_R11
    + /* rcx and r11 are already restored (see code above) */
    + RESTORE_C_REGS_EXCEPT_RCX_R11
    movq RSP(%rsp),%rsp
    USERGS_SYSRET64
    CFI_RESTORE_STATE
    --
    1.8.1.4


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