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([5.180.139.246]) by smtp.gmail.com with ESMTPSA id 2adb3069b0e04-5a2c6cd3698sm3937616e87.77.2026.04.07.01.15.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 07 Apr 2026 01:15:44 -0700 (PDT) From: "Denis M. Karpov" To: rppt@kernel.org, akpm@linux-foundation.org, Liam.Howlett@oracle.com, ljs@kernel.org Cc: vbabka@kernel.org, jannh@google.com, peterx@redhat.com, pfalcato@suse.de, brauner@kernel.org, viro@zeniv.linux.org.uk, jack@suse.cz, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, "Denis M . Karpov" Subject: [RFC PATCH] userfaultfd: allow registration of ranges below mmap_min_addr Date: Tue, 7 Apr 2026 11:14:42 +0300 Message-ID: <20260407081442.6256-1-komlomal@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit The current implementation of validate_range() in fs/userfaultfd.c performs a hard check against mmap_min_addr without considering capabilities, but the mmap() syscall uses security_mmap_addr() which allows privileged processes (with CAP_SYS_RAWIO) to map below mmap_min_addr. Furthermore, security_mmap_addr()->cap_mmap_addr() uses dac_mmap_min_addr variable which can be changed with /proc/sys/vm/mmap_min_addr. Because userfaultfd uses a different check, UFFDIO_REGISTER may fail with -EINVAL for valid memory areas that were successfully mapped below mmap_min_addr even with appropriate capabilities. This prevents apps like binary compilers from using UFFD for valid memory regions mapped by application. Replace the rigid mmap_min_addr check with security_mmap_addr() to align userfaultfd with the standard kernel memory mapping security policy. Signed-off-by: Denis M. Karpov --- Initial RFC following the discussion on the [BUG] thread. Link: https://lore.kernel.org/all/CADtiZd0tWysx5HMCUnOXfSHB7PXAuXg1Mh4eY_hUmH29S=sejg@mail.gmail.com/ --- fs/userfaultfd.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index bdc84e521..dbfe5b2a0 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -1238,15 +1238,13 @@ static __always_inline int validate_unaligned_range( return -EINVAL; if (!len) return -EINVAL; - if (start < mmap_min_addr) - return -EINVAL; if (start >= task_size) return -EINVAL; if (len > task_size - start) return -EINVAL; if (start + len <= start) return -EINVAL; - return 0; + return security_mmap_addr(start); } static __always_inline int validate_range(struct mm_struct *mm, -- 2.47.3