[PATCH v2] entry: Fix seccomp bypass after ptrace with TSYNC

[Jinjie Ruan]

Sashiko review pointed out the following issue.

If a thread is stopped in syscall_trace_enter() for ptrace, another
thread can install a seccomp filter with SECCOMP_FILTER_FLAG_TSYNC
(e.g., via seccomp_attach_filter()). This will successfully set
SYSCALL_WORK_SECCOMP on the stopped thread, but syscall_trace_enter()
evaluates a cached 'work' variable sampled on entry. Consequently,
the subsequent check for SYSCALL_WORK_SECCOMP misses the newly
assigned flag, and the filter is silently bypassed.

This race condition could allow an unprivileged process to execute
a prohibited system call (e.g., execve) that the newly installed filter
was intended to block, especially since the tracer might have modified
the system call number during the ptrace stop.

Fix this by re-reading the syscall_work flags after ptrace handling,
so that any new SYSCALL_WORK_SECCOMP flag set by another thread via
TSYNC during the ptrace stop is observed before the subsequent
seccomp check.

Cc: Kees Cook <kees@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Will Drewry <wad@xxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lore.kernel.org/all/20260629132914.1135C1F000E9@xxxxxxxxxxxxxxx/
Fixes: 142781e108b1 ("entry: Provide generic syscall entry functionality")
Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>
---
v2:
- Adopt lighter-weight approach suggested by Thomas: re-read syscall_work
  after ptrace instead of calling seccomp_permit_syscall() unconditionally.
- Update the commit message to reflect the actual implementation.
---
 include/linux/entry-common.h | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index 07d97def7dcb..299f13c78a6f 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -97,6 +97,9 @@ static __always_inline long syscall_trace_enter(struct pt_regs *regs, unsigned l
 		if (!arch_ptrace_report_syscall_permit_entry(regs) ||
 		    (work & SYSCALL_WORK_SYSCALL_EMU))
 			return -1L;
+
+		/* ptrace might have changed work flags */
+		work = READ_ONCE(current_thread_info()->syscall_work);
 	}
 
 	/* Do seccomp after ptrace, to catch any tracer changes. */
-- 
2.34.1