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Message-ID: <03206759-8d6a-a410-a8f9-d4678236bdbf@gmail.com>
Date: Wed, 19 Jul 2023 04:35:01 +0800
From: Celeste Liu <coelacanthushex@...il.com>
To: Björn Töpel <bjorn@...nel.org>,
Palmer Dabbelt <palmer@...osinc.com>,
Paul Walmsley <paul.walmsley@...ive.com>,
Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>,
Björn Töpel <bjorn@...osinc.com>,
Conor Dooley <conor.dooley@...rochip.com>,
linux-riscv@...ts.infradead.org
Cc: linux-kernel@...r.kernel.org,
Felix Yan <felixonmars@...hlinux.org>,
Ruizhe Pan <c141028@...il.com>,
Shiqi Zhang <shiqi@...c.iscas.ac.cn>
Subject: Re: [PATCH v2] riscv: entry: set a0 prior to
syscall_enter_from_user_mode
On 2023/7/19 03:35, Björn Töpel wrote:
> Celeste Liu <coelacanthushex@...il.com> writes:
>
>> When we test seccomp with 6.4 kernel, we found errno has wrong value.
>> If we deny NETLINK_AUDIT with EAFNOSUPPORT, after f0bddf50586d, we will
>> get ENOSYS instead. We got same result with 9c2598d43510 ("riscv: entry: Save a0
>> prior syscall_enter_from_user_mode()").
>>
>> After analysing code, we think that regs->a0 = -ENOSYS should be advanced before
>> syscall_enter_from_user_mode to fix this problem. In __seccomp_filter, when
>> seccomp rejected this syscall with specified errno, they will set a0 to return
>> number as syscall ABI, and then return -1. This return number is finally pass
>> as return number of syscall_enter_from_user_mode, and then is compared
>> with NR_syscalls after converted to ulong (so it will be ULONG_MAX).
>> The condition syscall < NR_syscalls will always be false, so regs->a0 = -ENOSYS
>> is always executable. It covered a0 set by seccomp, so we always get
>> ENOSYS when match seccomp RET_ERRNO rule.
>
> Isn't something like...
>
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index f910dfccbf5d..15a8b4898a6c 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -297,7 +297,7 @@ asmlinkage __visible __trap_section void do_trap_break(struct pt_regs *regs)
> asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
> {
> if (user_mode(regs)) {
> - ulong syscall = regs->a7;
> + long syscall = regs->a7;
>
> regs->epc += 4;
> regs->orig_a0 = regs->a0;
> @@ -306,7 +306,7 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>
> syscall = syscall_enter_from_user_mode(regs, syscall);
>
> - if (syscall < NR_syscalls)
> + if (syscall > -1 && syscall < NR_syscalls)
> syscall_handler(regs, syscall);
> else
> regs->a0 = -ENOSYS;
>
>
> ...easier to read?
>
>
> Björn
It seems that your change turn it back to the beginning. If syscall == -1,
it is supposed to go neither first nor else branch. It should do NOTHING.
However it was still a great idea. It may be better to use a set of if-statement
to clarify the logic.
diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
index f910dfccbf5d2..d0bd725244594 100644
--- a/arch/riscv/kernel/traps.c
+++ b/arch/riscv/kernel/traps.c
@@ -306,7 +306,9 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
syscall = syscall_enter_from_user_mode(regs, syscall);
- if (syscall < NR_syscalls)
+ if (syscall == -1)
+ // Do nothing
+ else if (syscall < NR_syscalls)
syscall_handler(regs, syscall);
else
regs->a0 = -ENOSYS;
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