• Sven Wegener's avatar
    x86_32, entry: Store badsys error code in %eax · 8142b215
    Sven Wegener authored
    Commit 554086d8 ("x86_32, entry: Do syscall exit work on badsys
    (CVE-2014-4508)") introduced a regression in the x86_32 syscall entry
    code, resulting in syscall() not returning proper errors for undefined
    syscalls on CPUs supporting the sysenter feature.
    
    The following code:
    
    > int result = syscall(666);
    > printf("result=%d errno=%d error=%s\n", result, errno, strerror(errno));
    
    results in:
    
    > result=666 errno=0 error=Success
    
    Obviously, the syscall return value is the called syscall number, but it
    should have been an ENOSYS error. When run under ptrace it behaves
    correctly, which makes it hard to debug in the wild:
    
    > result=-1 errno=38 error=Function not implemented
    
    The %eax register is the return value register. For debugging via ptrace
    the syscall entry code stores the complete register context on the
    stack. The badsys handlers only store the ENOSYS error code in the
    ptrace register set and do not set %eax like a regular syscall handler
    would. The old resume_userspace call chain contains code that clobbers
    %eax and it restores %eax from the ptrace registers afterwards. The same
    goes for the ptrace-enabled call chain. When ptrace is not used, the
    syscall return value is the passed-in syscall number from the untouched
    %eax register.
    
    Use %eax as the return value register in syscall_badsys and
    sysenter_badsys, like a real syscall handler does, and have the caller
    push the value onto the stack for ptrace access.
    Signed-off-by: default avatarSven Wegener <sven.wegener@stealer.net>
    Link: http://lkml.kernel.org/r/alpine.LNX.2.11.1407221022380.31021@titan.int.lan.stealer.netReviewed-and-tested-by: default avatarAndy Lutomirski <luto@amacapital.net>
    Cc: <stable@vger.kernel.org> # If 554086d8 is backported
    Signed-off-by: default avatarH. Peter Anvin <hpa@zytor.com>
    8142b215
entry_32.S 32.5 KB