From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-177.mta1.migadu.com (out-177.mta1.migadu.com [95.215.58.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 5548A263C8C for ; Wed, 8 Apr 2026 12:37:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.177 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775651843; cv=none; b=EeiNDDX0J2rQ68vT3+h/+LhlsLo0c8Nt4J5VG0eLGT/TYEK1BZGAuQkb5mbT2s/cjOav4mk7kpGfKw7gIn3rUVvO+Kt2ZmRsrdFYHRJ+MDduWEKW3d6TRLF4FCE+AsmAM7gKHGQC7CGe+xecTm7qyniEfGOVy/vDVNqNpEobi8k= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775651843; c=relaxed/simple; bh=NLwi39iG6x+4DNei3a8leGjXiKw3I9Rzak2FVdeN0d8=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=F7NOTwIjuwaQY+amRjhL4s1szrYffiJCHQYfuiwV0N+zbtsA6L4MfKyMY383urNk3M+F+USl5WOBUimHv6AWJuv2u+RZKrJdcOLGtrLDAPv2iRSS6GSeGoyppLnmNXW6LSkNzzzue8CLN/NMPLMcnoXdQ6hmFY8468uWgsQ48nk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=LojMvni9; arc=none smtp.client-ip=95.215.58.177 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="LojMvni9" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1775651830; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=w7ozxszcWAzUiiO7dnD+2XLganhUT/yvfmOrIFi0J4o=; b=LojMvni9LRcUFivfS2//FW6d4oquvN57vihlfqSaUW5XqAUIYTV/zoRXKgzs+sk1dtwBY3 45254wkgwY7gkR/4eSAIZrf9nWHh+UlKN3vMXpTdOgS0uMkyQqYBaMlvMVPfi/FCBpSrnq j6fbEfOSQU7/VVrzdgvjzRlJd3kGoWM= From: Usama Arif To: "Denis M. Karpov" Cc: Usama Arif , rppt@kernel.org, akpm@linux-foundation.org, Liam.Howlett@oracle.com, ljs@kernel.org, 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 Subject: Re: [RFC PATCH] userfaultfd: allow registration of ranges below mmap_min_addr Date: Wed, 8 Apr 2026 05:36:59 -0700 Message-ID: <20260408123700.1596800-1-usama.arif@linux.dev> In-Reply-To: <20260407081442.6256-1-komlomal@gmail.com> References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT On Tue, 7 Apr 2026 11:14:42 +0300 "Denis M. Karpov" wrote: > 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); Is this introducing an ABI change? The old code returned -EINVAL when start was below mmap_min_addr. The new code calls security_mmap_addr() which returns -EPERM when the caller lacks CAP_SYS_RAWIO. Existing userspace callers checking specifically for -EINVAL would see different behavior start is below mmap_min_addr. > } > > static __always_inline int validate_range(struct mm_struct *mm, > -- > 2.47.3 > >