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    Subject[PATCH] i386/audit: stop scribbling on the stack frame
    git commit b4f0d3755c5e9cc86292d5fd78261903b4f23d4a was very very dumb.
    It was writing over %esp/pt_regs semi-randomly on i686 with the expected
    "system can't boot" results. As noted in:

    https://bugs.freedesktop.org/show_bug.cgi?id=85277

    This patch stops fscking with pt_regs. Instead it sets up the registers
    for the call to __audit_syscall_entry in the most obvious conceivable
    way. It then does just a tiny tiny touch of magic. We need to get what
    started in PT_EDX into 0(%esp) and PT_ESI into 4(%esp). This is as easy
    as a pair of pushes using the values still in those registers.

    After the call to __audit_syscall_entry all we need to do is get that
    now useless junk off the stack (pair of pops) and reload %eax with the
    original syscall so other stuff can keep going about it's business.

    Reported-by: Paulo Zanoni <przanoni@gmail.com>
    Signed-off-by: Eric Paris <eparis@redhat.com>
    Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: "H. Peter Anvin" <hpa@zytor.com>
    Cc: x86@kernel.org
    Cc: linux-kernel@vger.kernel.org
    Cc: linux-audit@redhat.com

    ---
    On 14/10/25, Thomas Gleixner wrote:
    > Why are we grabbing that from the stack? AFAICT all arguments are in
    > the registers still.

    Right, re-arranging the instructions slightly to avoid overwriting %edx
    with %ebx before needing it to push onto the stack, how does this look?

    arch/x86/kernel/entry_32.S | 15 +++++++--------
    1 file changed, 7 insertions(+), 8 deletions(-)

    diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S
    index b553ed8..344b63f 100644
    --- a/arch/x86/kernel/entry_32.S
    +++ b/arch/x86/kernel/entry_32.S
    @@ -447,15 +447,14 @@ sysenter_exit:
    sysenter_audit:
    testl $(_TIF_WORK_SYSCALL_ENTRY & ~_TIF_SYSCALL_AUDIT),TI_flags(%ebp)
    jnz syscall_trace_entry
    - addl $4,%esp
    - CFI_ADJUST_CFA_OFFSET -4
    - movl %esi,4(%esp) /* 5th arg: 4th syscall arg */
    - movl %edx,(%esp) /* 4th arg: 3rd syscall arg */
    - /* %ecx already in %ecx 3rd arg: 2nd syscall arg */
    - movl %ebx,%edx /* 2nd arg: 1st syscall arg */
    - /* %eax already in %eax 1st arg: syscall number */
    + /* movl PT_ECX(%esp), %ecx already set, a1: 3nd arg to audit */
    + /* movl PT_EAX(%esp), %eax already set, syscall number: 1st arg to audit */
    + pushl_cfi %esi /* a3: 5th arg */
    + pushl_cfi %edx /* a2: 4th arg */
    + movl %ebx, %edx /* ebx/a0: 2nd arg to audit */
    call __audit_syscall_entry
    - pushl_cfi %ebx
    + popl_cfi %ecx /* get that remapped edx off the stack */
    + popl_cfi %ecx /* get that remapped esi off the stack */
    movl PT_EAX(%esp),%eax /* reload syscall number */
    jmp sysenter_do_call

    - RGB

    --
    Richard Guy Briggs <rbriggs@redhat.com>
    Senior Software Engineer, Kernel Security, AMER ENG Base Operating Systems, Red Hat
    Remote, Ottawa, Canada
    Voice: +1.647.777.2635, Internal: (81) 32635, Alt: +1.613.693.0684x3545


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