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Fri, 26 Dec 2025 11:25:09 -0800 (PST) Received: from localhost ([188.164.161.46]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-59a185d5db1sm6968592e87.18.2025.12.26.11.25.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 26 Dec 2025 11:25:08 -0800 (PST) From: Artur Sultangirov To: peterz@infradead.org, mingo@redhat.com Cc: Artur Sultangirov , mathieu.desnoyers@efficios.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, linux-kernel@vger.kernel.org Subject: [BUG] sched/mm_cid: NULL pointer dereference in sched_mm_cid_fork on ARM64 Date: Fri, 26 Dec 2025 22:25:05 +0300 Message-ID: <20251226192506.88593-1-za4emsu@gmail.com> X-Mailer: git-send-email 2.52.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Hi, I encountered a kernel panic on ARM64 (Apple M4 via QEMU in TCG mode) when executing a binary with a wrong architecture (x86_64 on aarch64). Instead of returning -ENOEXEC and exiting gracefully, the kernel crashes with a NULL pointer dereference in mutex_lock(), called from sched_mm_cid_fork(). **Analysis:** The crash happens because `sched_mm_cid_fork()` checks for `!mm` using `WARN_ON_ONCE()`, but does NOT return if `mm` is NULL. It proceeds to dereference `mm` to access `mm->mm_cid.mutex`, leading to a panic. Code snippet (kernel/sched/core.c:10572): WARN_ON_ONCE(!mm || t->mm_cid.cid != MM_CID_UNSET); guard(mutex)(&mm->mm_cid.mutex); // <--- Dereference happens here **Steps to reproduce:** 1. Boot an ARM64 kernel (reproduced on 6.19-rc2, master branch). 2. Attempt to execute an x86_64 binary as init (or via execve). 3. The execve fails, `mm` becomes NULL (or uninitialized), but the scheduler hook is called. **Stack trace:** [ 0.911926] Run /bad_init as init process [ 0.916411] ------------[ cut here ]------------ [ 0.916480] WARNING: kernel/sched/core.c:10569 at sched_mm_cid_fork+0x3c/0x35c, CPU#3: swapper/0/1 [ 0.917233] Modules linked in: [ 0.917704] CPU: 3 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.19.0-rc2-00055-gccd1cdca5cd4-dirty #7 PREEMPT [ 0.917866] Hardware name: linux,dummy-virt (DT) [ 0.918118] pc : sched_mm_cid_fork+0x3c/0x35c [ 0.918185] lr : sched_mm_cid_after_execve+0x10/0x1c ... [ 0.919284] Call trace: [ 0.919387] sched_mm_cid_fork+0x3c/0x35c (P) [ 0.919522] sched_mm_cid_after_execve+0x10/0x1c [ 0.919588] bprm_execve+0x32c/0x470 [ 0.919641] kernel_execve+0x25c/0x274 ... [ 0.920548] Unable to handle kernel NULL pointer dereference at virtual address 00000000000000e0 [ 0.921346] Internal error: Oops: 000000009600006b [#1] SMP ... [ 0.923475] Call trace: [ 0.923503] mutex_lock+0x18/0x54 (P) [ 0.923552] sched_mm_cid_fork+0x4c/0x35c [ 0.923610] sched_mm_cid_after_execve+0x10/0x1c [ 0.923662] bprm_execve+0x32c/0x470 ... [ 0.925595] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b ]--- **Suggestion:** The function should probably return immediately if `mm` is NULL, similar to how it is handled in other scheduler hooks. Best regards, Artur Sultangirov