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    Subject[PATCH 5.4 20/66] x86/entry/32: Use %ss segment where required
    Date
    From: Andy Lutomirski <luto@kernel.org>

    commit 4c4fd55d3d59a41ddfa6ecba7e76928921759f43 upstream.

    When re-building the IRET frame we use %eax as an destination %esp,
    make sure to then also match the segment for when there is a nonzero
    SS base (ESPFIX).

    [peterz: Changelog and minor edits]
    Fixes: 3c88c692c287 ("x86/stackframe/32: Provide consistent pt_regs")
    Signed-off-by: Andy Lutomirski <luto@kernel.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: stable@kernel.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/entry/entry_32.S | 19 ++++++++++++++-----
    1 file changed, 14 insertions(+), 5 deletions(-)

    --- a/arch/x86/entry/entry_32.S
    +++ b/arch/x86/entry/entry_32.S
    @@ -210,6 +210,8 @@
    /*
    * The high bits of the CS dword (__csh) are used for CS_FROM_*.
    * Clear them in case hardware didn't do this for us.
    + *
    + * Be careful: we may have nonzero SS base due to ESPFIX.
    */
    andl $0x0000ffff, 3*4(%esp)

    @@ -263,6 +265,13 @@
    .endm

    .macro IRET_FRAME
    + /*
    + * We're called with %ds, %es, %fs, and %gs from the interrupted
    + * frame, so we shouldn't use them. Also, we may be in ESPFIX
    + * mode and therefore have a nonzero SS base and an offset ESP,
    + * so any attempt to access the stack needs to use SS. (except for
    + * accesses through %esp, which automatically use SS.)
    + */
    testl $CS_FROM_KERNEL, 1*4(%esp)
    jz .Lfinished_frame_\@

    @@ -276,20 +285,20 @@
    movl 5*4(%esp), %eax # (modified) regs->sp

    movl 4*4(%esp), %ecx # flags
    - movl %ecx, -4(%eax)
    + movl %ecx, %ss:-1*4(%eax)

    movl 3*4(%esp), %ecx # cs
    andl $0x0000ffff, %ecx
    - movl %ecx, -8(%eax)
    + movl %ecx, %ss:-2*4(%eax)

    movl 2*4(%esp), %ecx # ip
    - movl %ecx, -12(%eax)
    + movl %ecx, %ss:-3*4(%eax)

    movl 1*4(%esp), %ecx # eax
    - movl %ecx, -16(%eax)
    + movl %ecx, %ss:-4*4(%eax)

    popl %ecx
    - lea -16(%eax), %esp
    + lea -4*4(%eax), %esp
    popl %eax
    .Lfinished_frame_\@:
    .endm

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