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SubjectRe: FW: i386 Linux kernel DoS (clarification)
On Wed, Nov 13, 2002 at 09:10:14PM +0000, Alan Cox wrote:
> On Wed, 2002-11-13 at 20:36, Petr Vandrovec wrote:
> > 2.5.47-current-bk, run as mere user: Kernel panic: Attempted to kill init!
> > Next time I'll trust you.
> It does the lcall
> The lcall takes an exception
> The exception (TF) has NT set
> iret returns via the task linkage
>
> I think just clearing the NT bit in both lcall path _and_ in the TF
> exception handler does the trick.

This fixes it for me. I'll have to look at ia32 manual at home, why I
must do pushl %eax & popfl, as NT should be already cleared by
do_debug(). I probably miss something obvious, but I do not think that
adding these three instructions into lcall7/27 fastpath is acceptable.

Without pushl %eax & popfl it behaved much better than originally:
modprobe started, said that personality-1 does not exist, and then
system killed init (instead of killing init immediately).
Best regards,
Petr Vandrovec
vandrove@vc.cvut.cz


diff -urN linux-2.5.47.dist/arch/i386/kernel/entry.S linux-2.5.47/arch/i386/kernel/entry.S
--- linux-2.5.47.dist/arch/i386/kernel/entry.S 2002-11-11 12:26:04.000000000 +0100
+++ linux-2.5.47/arch/i386/kernel/entry.S 2002-11-13 22:02:17.000000000 +0100
@@ -131,6 +131,9 @@
movl EIP(%esp), %eax # due to call gates, this is eflags, not eip..
movl CS(%esp), %edx # this is eip..
movl EFLAGS(%esp), %ecx # and this is cs..
+ andl $~NT_MASK, %eax
+ pushl %eax
+ popfl
movl %eax,EFLAGS(%esp) #
movl %edx,EIP(%esp) # Now we move them to their "normal" places
movl %ecx,CS(%esp) #
@@ -153,6 +156,9 @@
movl EIP(%esp), %eax # due to call gates, this is eflags, not eip..
movl CS(%esp), %edx # this is eip..
movl EFLAGS(%esp), %ecx # and this is cs..
+ andl $~NT_MASK, %eax
+ pushl %eax
+ popfl
movl %eax,EFLAGS(%esp) #
movl %edx,EIP(%esp) # Now we move them to their "normal" places
movl %ecx,CS(%esp) #
diff -urN linux-2.5.47.dist/arch/i386/kernel/traps.c linux-2.5.47/arch/i386/kernel/traps.c
--- linux-2.5.47.dist/arch/i386/kernel/traps.c 2002-11-11 12:26:02.000000000 +0100
+++ linux-2.5.47/arch/i386/kernel/traps.c 2002-11-13 21:54:26.000000000 +0100
@@ -636,7 +636,7 @@
return;

clear_TF:
- regs->eflags &= ~TF_MASK;
+ regs->eflags &= ~(TF_MASK|NT_MASK);
return;
}

-
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