On 2024-10-21 22:00, Björn Töpel wrote:
> Celeste!
>
> I'll pick up your thread [1] here, since there's code here! :-) Let's
> try to clear up/define the possible flow.
>
> The common/generic entry syscall_enter_from_user_mode{,_work}() says
> that a return value of -1 means that the syscall should be skipped.
>
> The RISC-V calling convention uses the same register for arg0/retval
> (a0). So, when a ptracer (PTRACE_SYSCALL+PTRACE_GETREGS). That means
> that the kernel cannot call into the tracer, *after* changing a0.
>
> The interesting flow for a syscall is roughly:
> 1. The exception/trap function
> 2. syscall_enter_from_user_mode() which might return -1, meaning that
> the syscall should be skipped. A tracer might have altered the
> regs. More importantly, if it's -1 the kernel cannot change the
> return value, because seccomp filtering might already done that.
> 3. If it's a valid syscall, perform the syscall.
>
> Now some scenarios:
> * A user does a valid syscall.
> * A user does an invalid syscall(-1)
> * A user does an invalid syscall(!= -1)
>
> Then there're the tracing variants of those, and that's where we go get
> trouble.
>
> Now the bugs we've seen in RISC-V:
>
> 1. strace "tracing": Requires that regs->a0 is not tampered with prior
> ptrace notification
>
> E.g.:
> | # ./strace /
> | execve("/", ["/"], 0x7ffffaac3890 /* 21 vars */) = -1 EACCES (Permission denied)
> | ./strace: exec: Permission denied
> | +++ exited with 1 +++
> | # ./disable_ptrace_get_syscall_info ./strace /
> | execve(0xffffffffffffffda, ["/"], 0x7fffd893ce10 /* 21 vars */) = -1 EACCES (Permission denied)
> | ./strace: exec: Permission denied
> | +++ exited with 1 +++
>
> In the second case, arg0 is prematurely set to -ENOSYS
> (0xffffffffffffffda).
>
> 2. strace "syscall tampering": Requires that ENOSYS is returned for
> syscall(-1), and not skipped w/o a proper return value.
>
> E.g.:
> | ./strace -a0 -ewrite -einject=write:error=enospc echo helloject=write:error=enospc echo hello
>
> Here, strace expects that injecting -1, would result in a ENOSYS.
>
> 3. seccomp filtering: Requires that the a0 is not tampered to
>
> First 3 was broken (tampering with a0 after seccomp), then 2 was
> broken (not setting ENOSYS for -1), and finally 1 was broken (and
> still is!).
>
> Now for your patch:
>
> Celeste Liu via B4 Relay <devnull+CoelacanthusHex.gmail.com@xxxxxxxxxx>
> writes:
>
>> From: Celeste Liu <CoelacanthusHex@xxxxxxxxx>
>>
>> The return value of syscall_enter_from_user_mode() is always -1 when the
>> syscall was filtered. We can't know whether syscall_nr is -1 when we get -1
>> from syscall_enter_from_user_mode(). And the old syscall variable is
>> unusable because syscall_enter_from_user_mode() may change a7 register.
>> So get correct syscall number from syscall_get_nr().
>>
>> So syscall number part of return value of syscall_enter_from_user_mode()
>> is completely useless. We can remove it from API and require caller to
>> get syscall number from syscall_get_nr(). But this change affect more
>> architectures and will block more time. So we split it into another
>> patchset to avoid block this fix. (Other architectures can works
>> without this change but riscv need it, see Link: tag below)
>>
>> Fixes: 61119394631f ("riscv: entry: always initialize regs->a0 to -ENOSYS")
>> Reported-by: Andrea Bolognani <abologna@xxxxxxxxxx>
>> Closes: https://github.com/strace/strace/issues/315
>> Link: https://lore.kernel.org/all/59505464-c84a-403d-972f-d4b2055eeaac@xxxxxxxxx/
>> Signed-off-by: Celeste Liu <CoelacanthusHex@xxxxxxxxx>
>> ---
>> arch/riscv/kernel/traps.c | 13 ++++++++++---
>> 1 file changed, 10 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
>> index 51ebfd23e0076447518081d137102a9a11ff2e45..3125fab8ee4af468ace9f692dd34e1797555cce3 100644
>> --- a/arch/riscv/kernel/traps.c
>> +++ b/arch/riscv/kernel/traps.c
>> @@ -316,18 +316,25 @@ void do_trap_ecall_u(struct pt_regs *regs)
>> {
>> if (user_mode(regs)) {
>> long syscall = regs->a7;
>> + long res;
>>
>> regs->epc += 4;
>> regs->orig_a0 = regs->a0;
>> - regs->a0 = -ENOSYS;
>>
>> riscv_v_vstate_discard(regs);
>>
>> - syscall = syscall_enter_from_user_mode(regs, syscall);
>> + res = syscall_enter_from_user_mode(regs, syscall);
>> + /*
>> + * Call syscall_get_nr() again because syscall_enter_from_user_mode()
>> + * may change a7 register.
>> + */
>> + syscall = syscall_get_nr(current, regs);
>>
>> add_random_kstack_offset();
>>
>> - if (syscall >= 0 && syscall < NR_syscalls)
>> + if (syscall < 0 || syscall >= NR_syscalls)
>> + regs->a0 = -ENOSYS;
>> + else if (res != -1)
>> syscall_handler(regs, syscall);
>
> Here we can perform the syscall, even if res is -1. E.g., if this path
> [2] is taken, we might have a valid syscall number in a7, but the
> syscall should not be performed.
I may misunderstand what you said, but I can't see the issue you pointed.
A syscall is performed iff
1) syscall number in a7 must be valid, so it can reach "else if" branch.
2) res != -1, so syscall_enter_from_user_mode() doesn't return -1 to
inform the syscall should be skipped.
If [2] path is taken, syscall_trace_enter() will return -1 and pass through
syscall_enter_from_user_mode() so res == -1, the syscall will not be performed.
>
> Also, one reason for the generic entry is so that it should be less
> work. Here, you pull (IMO) details that belong to the common entry
> implementation/API up the common entry user. Wdyt about pushing it down
> to common entry? Something like:
Yeah, we can. But I pull it out of common entry to get more simple API semantic:
1. syscall_enter_from_user_mode() will do two things:
1) the return value is only to inform whether the syscall should be skipped.
2) regs will be modified by filters (seccomp or ptrace and so on).
2. for common entry user, there is two informations: syscall number and
the return value of syscall_enter_from_user_mode() (called is_skipped below).
so there is three situations:
1) if syscall number is invalid, the syscall should not be performed, and
we set a0 to -ENOSYS to inform userspace the syscall doesn't exist.
2) if syscall number is valid, is_skipped will be used:
a) if is_skipped is -1, which means there are some filters reject this syscall,
so the syscall should not performed. (Of course, we can use bool instead to
get better semantic)
b) if is_skipped != -1, which means the filters approved this syscall,
so we invoke syscall handler with modified regs.
In your design, the logical condition is not obvious. Why syscall_enter_from_user_mode()
informed the syscall will be skipped but the syscall handler will be called
when syscall number is invalid? The users need to think two things to get result:
a) -1 means skip
b) -1 < 0 in signed integer, so the skip condition is always a invalid syscall number.
In may way, the users only need to think one thing: The syscall_enter_from_user_mode()
said -1 means the syscall should not be performed, so use it as a condition of reject
directly. They just need to combine the informations that they get from API as the
condition of control flow.
>
> --8<--
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index 51ebfd23e007..66fded8e4b60 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -319,7 +319,6 @@ void do_trap_ecall_u(struct pt_regs *regs)
>
> regs->epc += 4;
> regs->orig_a0 = regs->a0;
> - regs->a0 = -ENOSYS;
>
> riscv_v_vstate_discard(regs);
>
> @@ -329,6 +328,8 @@ void do_trap_ecall_u(struct pt_regs *regs)
>
> if (syscall >= 0 && syscall < NR_syscalls)
> syscall_handler(regs, syscall);
> + else if (syscall != -1)
> + regs->a0 = -ENOSYS;
>
> /*
> * Ultimately, this value will get limited by KSTACK_OFFSET_MAX(),
> diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
> index 1e50cdb83ae5..9b69c2ad4f12 100644
> --- a/include/linux/entry-common.h
> +++ b/include/linux/entry-common.h
> @@ -14,6 +14,7 @@
> #include <linux/kmsan.h>
>
> #include <asm/entry-common.h>
> +#include <asm/syscall.h>
>
> /*
> * Define dummy _TIF work flags if not defined by the architecture or for
> @@ -166,6 +167,8 @@ static __always_inline long syscall_enter_from_user_mode_work(struct pt_regs *re
>
> if (work & SYSCALL_WORK_ENTER)
> syscall = syscall_trace_enter(regs, syscall, work);
> + else if (syscall == -1L)
> + syscall_set_return_value(current, regs, -ENOSYS, 0);
>
> return syscall;
> }
> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index 5b6934e23c21..99742aee5002 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -43,8 +43,10 @@ long syscall_trace_enter(struct pt_regs *regs, long syscall,
> /* Handle ptrace */
> if (work & (SYSCALL_WORK_SYSCALL_TRACE | SYSCALL_WORK_SYSCALL_EMU)) {
> ret = ptrace_report_syscall_entry(regs);
> - if (ret || (work & SYSCALL_WORK_SYSCALL_EMU))
> + if (ret || (work & SYSCALL_WORK_SYSCALL_EMU)) {
> + syscall_set_return_value(current, regs, -ENOSYS, 0);
> return -1L;
> + }
> }
>
> /* Do seccomp after ptrace, to catch any tracer changes. */
> @@ -66,6 +68,14 @@ long syscall_trace_enter(struct pt_regs *regs, long syscall,
> syscall = syscall_get_nr(current, regs);
> }
>
> + /*
> + * If we're not setting the return value here, the context is
> + * gone; For higher callers, -1 means that the syscall should
> + * be skipped.
> + */
> + if (syscall == -1L)
> + syscall_set_return_value(current, regs, -ENOSYS, 0);
> +
> syscall_enter_audit(regs, syscall);
>
> return ret ? : syscall;
> --8<--
>
> I did a quick test of the above, and it seems to cover all the previous
> bugs -- but who knows! ;-)
>
>
> Björn
>
> [1] https://lore.kernel.org/linux-riscv/59505464-c84a-403d-972f-d4b2055eeaac@xxxxxxxxx/
> [2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/entry/common.c#n47
