Messages in this thread | | | From | Andy Lutomirski <> | Subject | Re: pt_regs->ax == -ENOSYS | Date | Tue, 27 Apr 2021 16:51:06 -0700 |
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> On Apr 27, 2021, at 4:29 PM, Kees Cook <keescook@chromium.org> wrote: > > On Tue, Apr 27, 2021 at 03:58:03PM -0700, H. Peter Anvin wrote: >>> 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? > > FWIW, there is some confusion with how syscall_trac_enter() signals the > "skip syscall" condition (-1L), vs actually calling "syscall -1".
Fortunately there is not, and never will be, a syscall -1. But I agree that calling max syscall + 1 should behave identically to calling a nonexistent syscall in the middle of the table.
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