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Tue, 23 Dec 2025 02:56:07 -0800 (PST) From: Breno Leitao Date: Tue, 23 Dec 2025 02:55:44 -0800 Subject: [PATCH 2/2] kthread: Warn if mm_struct lacks user_ns in kthread_use_mm() Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20251223-efi_fix_619-v1-2-e0146b8b9d73@debian.org> References: <20251223-efi_fix_619-v1-0-e0146b8b9d73@debian.org> In-Reply-To: <20251223-efi_fix_619-v1-0-e0146b8b9d73@debian.org> To: Ard Biesheuvel , Catalin Marinas , Will Deacon Cc: linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, riel@surriel.com, puranjay@kernel.org, usamaarif642@gmail.com, Breno Leitao , kernel-team@meta.com X-Mailer: b4 0.15-dev-47773 X-Developer-Signature: v=1; a=openpgp-sha256; l=1268; i=leitao@debian.org; h=from:subject:message-id; bh=JVkzh6Mj6r18BxTCZYOIUvNWS/o6kmzf/4mUqtZjoM0=; b=owEBbQKS/ZANAwAIATWjk5/8eHdtAcsmYgBpSnVE+bi2ZoQldtZ7u8jeYGlbnlyMfIcy78o4u qgOXEr0OzmJAjMEAAEIAB0WIQSshTmm6PRnAspKQ5s1o5Of/Hh3bQUCaUp1RAAKCRA1o5Of/Hh3 be7LEACHOsztZael5tTqpe9CNoDWhqU8isXgl0v5yAwcqk8Fjm5MKLgC2z7XxxeuUgCxNF2dQba 3ZNyA7Xh/twezTl30dyVRJX+pW3qbfL5irGxa1AV5ua9kSUObxME9BJRk3AReZ0+v+/Cdv1NIbm hYeTxSZg0CBZuWtWvtB2JkNna3nH954ALoHtSiHdSzAI+JBIXvbwP/6bHrhzBioX8g9A+JRbkXb DZtAnS09cTmABI/gYH3ydpbK8zM1ATWLIH5cVVm6oGHT+/ocSpW98EuJdvwV04VcUr6S2wCha9v iULKzhEB7ZJJvA4e+f2lS8Wol22Ka52YSCv9WTUa1lJKhhR/JQi5niFZoN2A7+qD2oCyjUxbKj/ Z5W+R1fChfCj1Ak91hQCKiYh0QHa3Sy7q5Emgo8hvsEekSTwRd+H3lfZaSgZ2/pyGSwkWh6myh5 IxzvT0EWz83bMSv1DPXp0wEOdvLFBzRpo2lOEzrtjsIPoMx7v5V0L5p1XrLrt1IdDmV3MMs7Ijy yD2Mcu36gtbS0aukBFiGH/E57s2DMKGS3mFcvU57Ug4BQMo1eFcBRbAu9jOvdi6r9EUn0p6oaEb tyz6zNpBvpfoFJdwrZ5RMszVcrHK8lxuJqPrcbz6+RQcKL+A1lRn/H+9PA6wOcYnUaYvT63Su+f MnT0qjGWX+M7iVA== X-Developer-Key: i=leitao@debian.org; a=openpgp; fpr=AC8539A6E8F46702CA4A439B35A3939FFC78776D Add a WARN_ON_ONCE() check to detect mm_struct instances that are missing user_ns initialization when passed to kthread_use_mm(). When a kthread adopts an mm via kthread_use_mm(), LSM hooks and capability checks may access current->mm->user_ns for credential validation. If user_ns is NULL, this leads to a NULL pointer dereference crash. This was observed with efi_mm on arm64, where commit a5baf582f4c0 ("arm64/efi: Call EFI runtime services without disabling preemption") introduced kthread_use_mm(&efi_mm), but efi_mm lacked user_ns initialization, causing crashes during /proc access. Adding this warning helps catch similar bugs early during development rather than waiting for hard-to-debug NULL pointer crashes in production. Signed-off-by: Breno Leitao Acked-by: Rik van Riel --- kernel/kthread.c | 1 + 1 file changed, 1 insertion(+) diff --git a/kernel/kthread.c b/kernel/kthread.c index 99a3808d086f..39511dd2abc9 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -1599,6 +1599,7 @@ void kthread_use_mm(struct mm_struct *mm) =20 WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD)); WARN_ON_ONCE(tsk->mm); + WARN_ON_ONCE(!mm->user_ns); =20 /* * It is possible for mm to be the same as tsk->active_mm, but --=20 2.47.3