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([2405:201:682f:389d:52ce:671a:31ef:56a1]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-c7400000532sm4626033a12.27.2026.03.16.08.20.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 16 Mar 2026 08:20:08 -0700 (PDT) From: Deepanshu Kartikey To: mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, kees@kernel.org, akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, Liam.Howlett@oracle.com, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, brauner@kernel.org, oleg@redhat.com Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Deepanshu Kartikey , syzbot+bbe6b99feefc3a0842de@syzkaller.appspotmail.com, Deepanshu Kartikey Subject: [PATCH v3] kernel/fork: validate exit_signal in kernel_clone() Date: Mon, 16 Mar 2026 20:49:56 +0530 Message-ID: <20260316151956.563558-1-kartikey406@gmail.com> X-Mailer: git-send-email 2.43.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 7FBB940011 X-Stat-Signature: i9a4oempr55o4kcnyg9i9iuabdnx5uon X-Rspam-User: X-HE-Tag: 1773674410-76236 X-HE-Meta: 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 8tGB6lyp 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: When a child process exits, it sends exit_signal to its parent via do_notify_parent(). The clone() syscall constructs exit_signal as: (lower_32_bits(clone_flags) & CSIGNAL) CSIGNAL is 0xff, so values in the range 65-255 are possible. However, valid_signal() only accepts signals up to _NSIG (64 on x86_64). A non-zero non-valid exit_signal acts the same as exit_signal == 0: the parent process is not signaled when the child terminates. The syzkaller reproducer triggers this by calling clone() with flags=0x80, resulting in exit_signal = (0x80 & CSIGNAL) = 128, which exceeds _NSIG and is not a valid signal. The v1 of this patch added the check only in the clone() syscall handler, which is incomplete. kernel_clone() has other callers such as sys_ia32_clone() which would remain unprotected. Move the check to kernel_clone() to cover all callers. Since the valid_signal() check is now in kernel_clone() and covers all callers including clone3(), the same check in copy_clone_args_from_user() becomes redundant and is removed. The higher 32bits check for clone3() is kept as it is clone3() specific. Note that this is a user-visible change: previously, passing an invalid exit_signal to clone() was silently accepted. The man page for clone() does not document any defined behavior for invalid exit_signal values, so rejecting them with -EINVAL is the correct behavior. It is unlikely that any sane application relies on passing an invalid exit_signal. Reported-by: syzbot+bbe6b99feefc3a0842de@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=bbe6b99feefc3a0842de Tested-by: syzbot+bbe6b99feefc3a0842de@syzkaller.appspotmail.com Link: https://lore.kernel.org/all/20260307064202.353405-1-kartikey406@gmail.com/T/ [v1] Link: https://lore.kernel.org/all/20260316104536.558108-1-kartikey406@gmail.com/T/ [v2] Fixes: 3f2c788a1314 ("fork: prevent accidental access to clone3 features") Signed-off-by: Deepanshu Kartikey --- v3: - Remove now redundant valid_signal() check from copy_clone_args_from_user() since kernel_clone() now covers all callers including clone3() - Update changelog to remove WARN_ON mention and describe actual bug behavior instead v2: - Move valid_signal() check from clone() syscall handler to kernel_clone() to cover all callers including sys_ia32_clone() - Update changelog to document user-visible behavior change --- kernel/fork.c | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/kernel/fork.c b/kernel/fork.c index 947a8dbce06a..73f7fe958a30 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2687,6 +2687,9 @@ pid_t kernel_clone(struct kernel_clone_args *args) (args->pidfd == args->parent_tid)) return -EINVAL; + if (!valid_signal(args->exit_signal)) + return -EINVAL; + /* * Determine whether and which event to report to ptracer. When * called from kernel_thread or CLONE_UNTRACED is explicitly @@ -2885,11 +2888,9 @@ static noinline int copy_clone_args_from_user(struct kernel_clone_args *kargs, return -EINVAL; /* - * Verify that higher 32bits of exit_signal are unset and that - * it is a valid signal + * Verify that higher 32bits of exit_signal are unset */ - if (unlikely((args.exit_signal & ~((u64)CSIGNAL)) || - !valid_signal(args.exit_signal))) + if (unlikely(args.exit_signal & ~((u64)CSIGNAL))) return -EINVAL; if ((args.flags & CLONE_INTO_CGROUP) && -- 2.43.0