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Message-ID: <alpine.LNX.2.11.1407202330420.31021@titan.int.lan.stealer.net> Date: Sun, 20 Jul 2014 23:33:50 +0200 (CEST) From: Sven Wegener <sven.wegener@...aler.net> To: "H. Peter Anvin" <hpa@...or.com> cc: Richard Weinberger <richard@....at>, X86 ML <x86@...nel.org>, Eric Paris <eparis@...hat.com>, Linux Kernel <linux-kernel@...r.kernel.org>, Steven Rostedt <rostedt@...dmis.org>, Borislav Petkov <bp@...en8.de>, Toralf Förster <toralf.foerster@....de>, stable@...r.kernel.org, Roland McGrath <roland@...hat.com>, Andy Lutomirski <luto@...capital.net>, Josh Boyer <jwboyer@...oraproject.org> Subject: [PATCH] x86_32, entry: store badsys error code in %eax Commit 554086d ("x86_32, entry: Do syscall exit work on badsys (CVE-2014-4508)") introduced a subtle regression in the x86_32 syscall entry code, resulting in syscall() not returning proper errors for non-existing syscalls on CPUs not 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 %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: Sven Wegener <sven.wegener@...aler.net> --- arch/x86/kernel/entry_32.S | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/arch/x86/kernel/entry_32.S b/arch/x86/kernel/entry_32.S index dbaa23e..0d0c9d4 100644 --- a/arch/x86/kernel/entry_32.S +++ b/arch/x86/kernel/entry_32.S @@ -425,8 +425,8 @@ sysenter_do_call: cmpl $(NR_syscalls), %eax jae sysenter_badsys call *sys_call_table(,%eax,4) - movl %eax,PT_EAX(%esp) sysenter_after_call: + movl %eax,PT_EAX(%esp) LOCKDEP_SYS_EXIT DISABLE_INTERRUPTS(CLBR_ANY) TRACE_IRQS_OFF @@ -502,6 +502,7 @@ ENTRY(system_call) jae syscall_badsys syscall_call: call *sys_call_table(,%eax,4) +syscall_after_call: movl %eax,PT_EAX(%esp) # store the return value syscall_exit: LOCKDEP_SYS_EXIT @@ -675,12 +676,12 @@ syscall_fault: END(syscall_fault) syscall_badsys: - movl $-ENOSYS,PT_EAX(%esp) - jmp syscall_exit + movl $-ENOSYS,%eax + jmp syscall_after_call END(syscall_badsys) sysenter_badsys: - movl $-ENOSYS,PT_EAX(%esp) + movl $-ENOSYS,%eax jmp sysenter_after_call END(syscall_badsys) CFI_ENDPROC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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