Messages in this thread | | | Date | Thu, 25 Sep 2014 18:00:05 -0500 | From | Chuck Ebbert <> | Subject | Re: [PATCH] x86 : Ensure X86_FLAGS_NT is cleared on syscall entry |
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On Thu, 25 Sep 2014 12:42:51 -0700 Anish Bhatt <anish@chelsio.com> wrote:
> The MSR_SYSCALL_MASK, which is responsible for clearing specific EFLAGS on > syscall entry, should also clear the nested task (NT) flag to be safe from > userspace injection. Without this fix the application segmentation > faults on syscall return because of the changed meaning of the IRET > instruction. > > Further details can be seen here https://bugs.winehq.org/show_bug.cgi?id=33275 > > Signed-off-by: Anish Bhatt <anish@chelsio.com> > Signed-off-by: Sebastian Lackner <sebastian@fds-team.de> > --- > arch/x86/kernel/cpu/common.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c > index e4ab2b4..3126558 100644 > --- a/arch/x86/kernel/cpu/common.c > +++ b/arch/x86/kernel/cpu/common.c > @@ -1184,7 +1184,7 @@ void syscall_init(void) > /* Flags to clear on syscall */ > wrmsrl(MSR_SYSCALL_MASK, > X86_EFLAGS_TF|X86_EFLAGS_DF|X86_EFLAGS_IF| > - X86_EFLAGS_IOPL|X86_EFLAGS_AC); > + X86_EFLAGS_IOPL|X86_EFLAGS_AC|X86_EFLAGS_NT); > } > > /*
To demonstrate the bug this fixes, just strace this program, adapted from the linked bug report:
#include <stdio.h> #include <unistd.h>
int main() { asm volatile("pushfq \n\t" \ "pop %rax \n\t" \ "or $0x4000,%rax \n\t" \ "push %rax \n\t" \ "popfq \n\t"); sleep(1);
return 0; }
I ran strace under gdb and found it dies here with SIGSEGV:
0x00007ffff7a505c2 <+50>: mov $0xea,%eax 0x00007ffff7a505c7 <+55>: syscall => 0x00007ffff7a505c9 <+57>: cmp $0xfffffffffffff000,%rax
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