Re: pt_regs->ax == -ENOSYS

From: H. Peter Anvin
Date: Tue Apr 27 2021 - 18:58:35 EST


On 4/27/21 2:28 PM, Andy Lutomirski wrote:

On Apr 27, 2021, at 2:15 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote:

Trying to stomp out some possible cargo cult programming?

In the process of going through the various entry code paths, I have to admit to being a bit confused why pt_regs->ax is set to -ENOSYS very early in the system call path.


It has to get set to _something_, and copying orig_ax seems perhaps silly. There could also be code that relies on ptrace poking -1 into the nr resulting in -ENOSYS.


Yeah. I obviously ran into this working on the common entry-exit code for FRED; the frame has annoyingly different formats because of this, and I wanted to avoid slowing down the system call path.

What is perhaps even more confusing is:

__visible noinstr void do_syscall_64(struct pt_regs *regs, unsigned long nr)
{
nr = syscall_enter_from_user_mode(regs, nr);

instrumentation_begin();
if (likely(nr < NR_syscalls)) {
nr = array_index_nospec(nr, NR_syscalls);
regs->ax = sys_call_table[nr](regs);
#ifdef CONFIG_X86_X32_ABI
} else if (likely((nr & __X32_SYSCALL_BIT) &&
(nr & ~__X32_SYSCALL_BIT) < X32_NR_syscalls)) {
nr = array_index_nospec(nr & ~__X32_SYSCALL_BIT,
X32_NR_syscalls);
regs->ax = x32_sys_call_table[nr](regs);
#endif
}
instrumentation_end();
syscall_exit_to_user_mode(regs);
}
#endif

Now, unless I'm completely out to sea, it seems to me that if syscall_enter_from_user_mode() changes the system call number to an invalid number and pt_regs->ax to !-ENOSYS then the system call will return a different value(!) depending on if it is out of range for the table (whatever was poked into pt_regs->ax) or if it corresponds to a hole in the table. This seems to me at least to be The Wrong Thing.

I think you’re right.


Calling regs->ax = sys_ni_syscall() in an else clause would arguably be the right thing here, except possibly in the case where nr (or (int)nr, see below) == -1 or < 0.

I think the check should be -1 for 64 bit but (u32)nr == (u32)-1 for the 32-bit path. Does that seem reasonable?

I'm thinking overall that depending on 64-bit %rax is once again a mistake; I realize that the assembly code that did that kept breaking because people messed with it, but we still have:

/*
* Only the low 32 bits of orig_ax are meaningful, so we return int.
* This importantly ignores the high bits on 64-bit, so comparisons
* sign-extend the low 32 bits.
*/
static inline int syscall_get_nr(struct task_struct *task, struct pt_regs *regs)
{
return regs->orig_ax;
}

"Different interpretation of the same data" is a notorious security trap. Zero-extending orig_ax would cause different behavior on 32 and 64 bits and differ from the above, so I'm thinking that just once and for all defining the system call number as a signed int for all the x86 ABIs would be the sanest.

It still doesn't really answer the question if "movq $-1,%rax; syscall" or "movl $-1,%eax; syscall" could somehow cause bad things to happen, though, which makes me a little bit nervous still.

-hpa