Messages in this thread Patch in this message | | | From | "H. Peter Anvin" <> | Subject | [PATCH v3 3/4] x86/syscall: treat out of range and gap system calls the same | Date | Fri, 14 May 2021 18:10:14 -0700 |
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From: "H. Peter Anvin (Intel)" <hpa@zytor.com>
The current 64-bit system call entry code treats out-of-range system calls differently than system calls that map to a hole in the system call table. This is visible to the user if system calls are intercepted via ptrace or seccomp and the return value (regs->ax) is modified: in the former case, the return value is preserved, and in the latter case, sys_ni_syscall() is called and the return value is forced to -ENOSYS.
For example, before this patch, 335 for x86-64 would force the exit code to be set to -ENOSYS even if poked by ptrace, but 548 would not, creating an observable difference between an out of range system call and a system call number that falls outside the range of the tables.
The API spec in <asm-generic/syscalls.h> is very clear that only (int)-1 is the non-system-call sentinel value, so make the system call behavior consistent by calling sys_ni_syscall() for all invalid system call numbers except for -1.
Although currently sys_ni_syscall() simply returns -ENOSYS, calling it explicitly is friendly for tracing and future possible extensions, and as this is an error path there is no reason to optimize it.
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com> --- arch/x86/entry/common.c | 4 ++++ 1 file changed, 4 insertions(+)
diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c index 00da0f5420de..f51bc17262db 100644 --- a/arch/x86/entry/common.c +++ b/arch/x86/entry/common.c @@ -52,6 +52,8 @@ __visible noinstr void do_syscall_64(struct pt_regs *regs, unsigned long nr) X32_NR_syscalls); regs->ax = x32_sys_call_table[nr](regs); #endif + } else if (unlikely((int)nr != -1)) { + regs->ax = __x64_sys_ni_syscall(regs); } instrumentation_end(); syscall_exit_to_user_mode(regs); @@ -76,6 +78,8 @@ static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs, if (likely(nr < IA32_NR_syscalls)) { nr = array_index_nospec(nr, IA32_NR_syscalls); regs->ax = ia32_sys_call_table[nr](regs); + } else if (unlikely((int)nr != -1)) { + regs->ax = __ia32_sys_ni_syscall(regs); } } -- 2.31.1
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