Messages in this thread | | | Subject | Re: pt_regs->ax == -ENOSYS | From | "H. Peter Anvin" <> | Date | Tue, 27 Apr 2021 15:58:03 -0700 |
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On 4/27/21 2:28 PM, Andy Lutomirski wrote: > >> On Apr 27, 2021, at 2:15 PM, H. Peter Anvin <hpa@zytor.com> 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
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