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    Subject[PATCH 2.6.21 review I] [4/25] x86: kernel-mode faults pollute current->thead
    Date

    From: Jeff Dike <jdike@addtoit.com>

    Kernel-mode traps on x86_64 can pollute the trap information for a previous
    userspace trap for which the signal has not yet been delivered to the
    process.

    do_trap and do_general_protection set task->thread.error_code and .trapno
    for kernel traps. If a kernel-mode trap arrives between the arrival of a
    userspace trap and the delivery of the associated SISGEGV to the process,
    the process will get the kernel trap information in its sigcontext.

    This causes UML process segfaults, as the trapno that the UML kernel sees
    is 13, rather than the 14 for normal page faults. So, the UML kernel
    passes the SIGSEGV along to its process.

    I don't claim to fully understand the problem. On the one hand, a check in
    do_general_protection for a pending SIGSEGV turned up nothing. On the
    other hand, this patch fixed the UML process segfault problem.

    The patch below moves the setting of error_code and trapno so that that
    only happens in the case of userspace faults. As a side-effect, this
    should speed up kernel-mode fault handling a tiny bit.

    I looked at i386, and there is a similar situation. In this case, there is
    duplicate code setting task->thread.error_code and trapno. I deleted one,
    leaving the copy that runs in the case of a userspace fault.

    Signed-off-by: Jeff Dike <jdike@addtoit.com>
    Signed-off-by: Andi Kleen <ak@suse.de>
    Cc: Andi Kleen <ak@suse.de>
    Signed-off-by: Andrew Morton <akpm@osdl.org>
    ---

    arch/i386/kernel/traps.c | 8 +++-----
    arch/x86_64/kernel/traps.c | 12 ++++++------
    2 files changed, 9 insertions(+), 11 deletions(-)

    Index: linux/arch/i386/kernel/traps.c
    ===================================================================
    --- linux.orig/arch/i386/kernel/traps.c
    +++ linux/arch/i386/kernel/traps.c
    @@ -474,8 +474,6 @@ static void __kprobes do_trap(int trapnr
    siginfo_t *info)
    {
    struct task_struct *tsk = current;
    - tsk->thread.error_code = error_code;
    - tsk->thread.trap_no = trapnr;

    if (regs->eflags & VM_MASK) {
    if (vm86)
    @@ -487,6 +485,9 @@ static void __kprobes do_trap(int trapnr
    goto kernel_trap;

    trap_signal: {
    + tsk->thread.error_code = error_code;
    + tsk->thread.trap_no = trapnr;
    +
    if (info)
    force_sig_info(signr, info, tsk);
    else
    @@ -601,9 +602,6 @@ fastcall void __kprobes do_general_prote
    }
    put_cpu();

    - current->thread.error_code = error_code;
    - current->thread.trap_no = 13;
    -
    if (regs->eflags & VM_MASK)
    goto gp_in_vm86;

    Index: linux/arch/x86_64/kernel/traps.c
    ===================================================================
    --- linux.orig/arch/x86_64/kernel/traps.c
    +++ linux/arch/x86_64/kernel/traps.c
    @@ -581,10 +581,10 @@ static void __kprobes do_trap(int trapnr
    {
    struct task_struct *tsk = current;

    - tsk->thread.error_code = error_code;
    - tsk->thread.trap_no = trapnr;
    -
    if (user_mode(regs)) {
    + tsk->thread.error_code = error_code;
    + tsk->thread.trap_no = trapnr;
    +
    if (exception_trace && unhandled_signal(tsk, signr))
    printk(KERN_INFO
    "%s[%d] trap %s rip:%lx rsp:%lx error:%lx\n",
    @@ -682,10 +682,10 @@ asmlinkage void __kprobes do_general_pro

    conditional_sti(regs);

    - tsk->thread.error_code = error_code;
    - tsk->thread.trap_no = 13;
    -
    if (user_mode(regs)) {
    + tsk->thread.error_code = error_code;
    + tsk->thread.trap_no = 13;
    +
    if (exception_trace && unhandled_signal(tsk, SIGSEGV))
    printk(KERN_INFO
    "%s[%d] general protection rip:%lx rsp:%lx error:%lx\n",
    -
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