Re: [PATCH] audit: ia32entry.S drops useful return value sign bits

Eric Paris
2011-05-24T13:13:01Z

On 05/23/2011 09:19 PM, H. Peter Anvin wrote:
> On 05/23/2011 06:04 PM, Eric Paris wrote:
>> 1) The audit_syscall_exit function expects a long. But if you chop off
>> the upper 32 bits you can't tell positive from negative. Thus when it
>> prints to userspace using %ld we have a problem:
>> Aka printf("%ld", (long)(u32)(-13)) = "4294967283"
>> vs printf("%ld", (long)(-13)) = "-13"
>
> This seems like the fundamental design error.

Possibly so (I'm not convinced), but not a fixable problem given the bug
for bug compatibility requirements of the kernel. The syscall return
value (either rax or eax) is passed to audit_syscall_exit() which
believes it is a long (aka s64). It builds a string buffer using
sprintf("%ld") and then exports that buffer to userspace via a netlink
socket. That buffer gets dumped as a raw string into a file. Some
tools may later process the strings. Getting the right string into the
netlink socket is what I consider the unchangeable ABI. Prior to
5cbf1565f29eb57a this was all handled by normal 64bit C code which did
exactly what I'm describing here. It never needlessly truncated the
return code to 32 bits on ia32exit. Solving that regression is what I'm
fixing.

> You're missing something fundamental: if userspace is 32 bits, those
> bits don't even exist. If userspace is 64 bits (and it is possible for
> a 64-bit process to call the 32-bit entry point) those bits could at
> least theoretically contain bad information.

This is at syscall exit, in 64bit mode, so rax is going to contain a
64bit version of the return code. I'm not trying to hand 64 bit values
back to a 32 bit process. The code converts the return value using %ld
and then dumps it as a string to auditd. Even if auditd was 32bit, it's
not processing the string, just writing it to a file.

> It sounds like this code is broken in some very fundamental ways, and
> that you're trying to paper it over.

Obviously we agree there is a second problem not addressed in this patch
(that many arches uses +/- instead of >=-MAX_ERRNO) but the fact that we
have a regression in which the assembly removes the sign and then passes
the now truncated value to a function expecting a long is the problem of
this patch.

I could paper over the problem in the audit code, doing my own sign
craziness based on the arch, but I think the real problem is in the
assembly dropping information needlessly.....

-Eric