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    Subject[PATCH 2/3] vsyscall_64: allow SECCOMP_RET_TRACErs to skip
    Date
    Current quirky ptrace behavior with vsyscall and seccomp
    does not allow tracers to bypass the call. This change
    provides that ability by checking if orig_ax changed.

    Signed-off-by: Will Drewry <wad@chromium.org>
    ---
    arch/x86/kernel/vsyscall_64.c | 10 +++++++---
    1 file changed, 7 insertions(+), 3 deletions(-)

    diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
    index 5db36ca..5f9640c 100644
    --- a/arch/x86/kernel/vsyscall_64.c
    +++ b/arch/x86/kernel/vsyscall_64.c
    @@ -142,11 +142,15 @@ static int addr_to_vsyscall_nr(unsigned long addr)
    #ifdef CONFIG_SECCOMP
    static int vsyscall_seccomp(struct task_struct *tsk, int syscall_nr)
    {
    + int ret;
    if (!seccomp_mode(&tsk->seccomp))
    return 0;
    task_pt_regs(tsk)->orig_ax = syscall_nr;
    task_pt_regs(tsk)->ax = syscall_nr;
    - return __secure_computing(syscall_nr);
    + ret = __secure_computing(syscall_nr);
    + if (task_pt_regs(tsk)->orig_ax != syscall_nr)
    + return 1; /* ptrace syscall skip */
    + return ret;
    }
    #else
    #define vsyscall_seccomp(_tsk, _nr) 0
    @@ -278,9 +282,9 @@ bool emulate_vsyscall(struct pt_regs *regs, unsigned long address)
    current_thread_info()->sig_on_uaccess_error = prev_sig_on_uaccess_error;

    if (skip) {
    - if ((long)regs->ax <= 0L) /* seccomp errno emulation */
    + if ((long)regs->ax <= 0L || skip == 1) /* seccomp errno/trace */
    goto do_ret;
    - goto done; /* seccomp trace/trap */
    + goto done; /* seccomp trap */
    }

    if (ret == -EFAULT) {
    --
    1.7.9.5


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