Re: Crash when attaching uretprobes to processes running in Docker

Masami Hiramatsu
2025-01-14T10:05:25Z

On Tue, 14 Jan 2025 10:22:20 +0100
Jiri Olsa <olsajiri@gmail.com> wrote:

> On Sat, Jan 11, 2025 at 07:40:15PM +0100, Jiri Olsa wrote:
> > On Sat, Jan 11, 2025 at 02:25:37AM +1100, Aleksa Sarai wrote:
> > > On 2025-01-10, Eyal Birger <eyal.birger@gmail.com> wrote:
> > > > Hi,
> > > >
> > > > When attaching uretprobes to processes running inside docker, the attached
> > > > process is segfaulted when encountering the retprobe. The offending commit
> > > > is:
> > > >
> > > > ff474a78cef5 ("uprobe: Add uretprobe syscall to speed up return probe")
> > > >
> > > > To my understanding, the reason is that now that uretprobe is a system call,
> > > > the default seccomp filters in docker block it as they only allow a specific
> > > > set of known syscalls.
> > >
> > > FWIW, the default seccomp profile of Docker _should_ return -ENOSYS for
> > > uretprobe (runc has a bunch of ugly logic to try to guarantee this if
> > > Docker hasn't updated their profile to include it). Though I guess that
> > > isn't sufficient for the magic that uretprobe(2) does...
> > >
> > > > This behavior can be reproduced by the below bash script, which works before
> > > > this commit.
> > > >
> > > > Reported-by: Rafael Buchbinder <rafi@rbk.io>
> >
> > hi,
> > nice ;-) thanks for the report, the problem seems to be that uretprobe syscall
> > is blocked and uretprobe trampoline does not expect that
> >
> > I think we could add code to the uretprobe trampoline to detect this and
> > execute standard int3 as fallback to process uretprobe, I'm checking on that
>
> hack below seems to fix the issue, it's using rbx to signal that uretprobe
> syscall got executed, if not, trampoline does int3 and executes uretprobe
> handler in the old way
>
> unfortunately now the uretprobe trampoline size crosses the xol slot limit so
> will need to come up with some generic/arch code solution for that, code below
> is neglecting that for now
>
> jirka
>
>
> ---
> arch/x86/kernel/uprobes.c | 24 ++++++++++++++++++++++++
> include/linux/uprobes.h | 1 +
> kernel/events/uprobes.c | 10 ++++++++--
> 3 files changed, 33 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/uprobes.c b/arch/x86/kernel/uprobes.c
> index 5a952c5ea66b..b54863f6fa25 100644
> --- a/arch/x86/kernel/uprobes.c
> +++ b/arch/x86/kernel/uprobes.c
> @@ -315,14 +315,25 @@ asm (
> ".global uretprobe_trampoline_entry\n"
> "uretprobe_trampoline_entry:\n"
> "pushq %rax\n"
> + "pushq %rbx\n"
> "pushq %rcx\n"
> "pushq %r11\n"
> + "movq $1, %rbx\n"
> "movq $" __stringify(__NR_uretprobe) ", %rax\n"
> "syscall\n"
> ".global uretprobe_syscall_check\n"
> "uretprobe_syscall_check:\n"
> + "or %rbx,%rbx\n"
> + "jz uretprobe_syscall_return\n"
> "popq %r11\n"
> "popq %rcx\n"
> + "popq %rbx\n"
> + "popq %rax\n"
> + "int3\n"
> + "uretprobe_syscall_return:\n"
> + "popq %r11\n"
> + "popq %rcx\n"
> + "popq %rbx\n"
>
> /* The uretprobe syscall replaces stored %rax value with final
> * return address, so we don't restore %rax in here and just
> @@ -338,6 +349,16 @@ extern u8 uretprobe_trampoline_entry[];
> extern u8 uretprobe_trampoline_end[];
> extern u8 uretprobe_syscall_check[];
>
> +#define UINSNS_PER_PAGE (PAGE_SIZE/UPROBE_XOL_SLOT_BYTES)
> +
> +bool arch_is_uretprobe_trampoline(unsigned long vaddr)
> +{
> + unsigned long start = uprobe_get_trampoline_vaddr();
> + unsigned long end = start + 2*UINSNS_PER_PAGE;
> +
> + return vaddr >= start && vaddr < end;
> +}
> +
> void *arch_uprobe_trampoline(unsigned long *psize)
> {
> static uprobe_opcode_t insn = UPROBE_SWBP_INSN;
> @@ -418,6 +439,9 @@ SYSCALL_DEFINE0(uretprobe)
> regs->r11 = regs->flags;
> regs->cx = regs->ip;
>
> + /* zero rbx to signal trampoline that uretprobe syscall was executed */
> + regs->bx = 0;

Can we just return -ENOSYS as like as other syscall instead of
using rbx as a side channel?
We can carefully check the return address is not -ERRNO when set up
and reserve the -ENOSYS for this use case.

Thank you,

> +
> return regs->ax;
>
> sigill:
> diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
> index e0a4c2082245..dbde57a68a1b 100644
> --- a/include/linux/uprobes.h
> +++ b/include/linux/uprobes.h
> @@ -213,6 +213,7 @@ extern void arch_uprobe_copy_ixol(struct page *page, unsigned long vaddr,
> extern void uprobe_handle_trampoline(struct pt_regs *regs);
> extern void *arch_uprobe_trampoline(unsigned long *psize);
> extern unsigned long uprobe_get_trampoline_vaddr(void);
> +bool arch_is_uretprobe_trampoline(unsigned long vaddr);
> #else /* !CONFIG_UPROBES */
> struct uprobes_state {
> };
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index fa04b14a7d72..73df64109f38 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -1703,6 +1703,11 @@ void * __weak arch_uprobe_trampoline(unsigned long *psize)
> return &insn;
> }
>
> +bool __weak arch_is_uretprobe_trampoline(unsigned long vaddr)
> +{
> + return vaddr == uprobe_get_trampoline_vaddr();
> +}
> +
> static struct xol_area *__create_xol_area(unsigned long vaddr)
> {
> struct mm_struct *mm = current->mm;
> @@ -1725,8 +1730,9 @@ static struct xol_area *__create_xol_area(unsigned long vaddr)
>
> area->vaddr = vaddr;
> init_waitqueue_head(&area->wq);
> - /* Reserve the 1st slot for get_trampoline_vaddr() */
> + /* Reserve the first two slots for get_trampoline_vaddr() */
> set_bit(0, area->bitmap);
> + set_bit(1, area->bitmap);
> insns = arch_uprobe_trampoline(&insns_size);
> arch_uprobe_copy_ixol(area->page, 0, insns, insns_size);
>
> @@ -2536,7 +2542,7 @@ static void handle_swbp(struct pt_regs *regs)
> int is_swbp;
>
> bp_vaddr = uprobe_get_swbp_addr(regs);
> - if (bp_vaddr == uprobe_get_trampoline_vaddr())
> + if (arch_is_uretprobe_trampoline(bp_vaddr))
> return uprobe_handle_trampoline(regs);
>
> rcu_read_lock_trace();
> --
> 2.47.1
>

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>