From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-lj1-f172.google.com (mail-lj1-f172.google.com [209.85.208.172]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 48A6C39B958 for ; Wed, 8 Apr 2026 08:09:14 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=pass smtp.client-ip=209.85.208.172 ARC-Seal:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775635755; cv=pass; b=Mz8mlKWW5cnFRMztfRUGdmlGjXw9Bcu8/2eNQ9vbPNNv+KDRbS1orlH4+LcP/vNquLvc9dulySLxu//QTffWg8NOxKdMFTHr05B1njxD+3KZxyVrWaxSjFYkAFbw/LWdWS0K2/j0smz926uSMA3pc0GZ+k1JlUbvtH0umOCTt60= ARC-Message-Signature:i=2; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1775635755; c=relaxed/simple; bh=Anf0QJShzbULca+i+nbk1xkmotYAjvT0BLx6u0VnoS0=; h=MIME-Version:References:In-Reply-To:From:Date:Message-ID:Subject: To:Cc:Content-Type; b=b3AfAxVEE1VrmPv6UcG9f0wGthgOTr61g7iJoI+i2KP1NfwosO5/rDmNaF9innP519hbCbYlQkKvOeLbsL7GSJhronnPFHqGRV13Iz0+cl/PuzGVmpuunIMGtEbrz8XfpGyIZsSUVJ54jrBXX8zVuMwqunhyBIR6b/SHKP5dRtI= ARC-Authentication-Results:i=2; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=M50qj+b/; arc=pass smtp.client-ip=209.85.208.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="M50qj+b/" Received: by mail-lj1-f172.google.com with SMTP id 38308e7fff4ca-386b553c70eso49217001fa.0 for ; Wed, 08 Apr 2026 01:09:14 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1775635752; cv=none; d=google.com; s=arc-20240605; b=UIMth0jqskMKpt0egDlAS4+1Vhc1t4SW2G9rtwO24E/thGgATefjyuG9XdcgjYx1jO P+7i014UySiPmrupmh7VL8SgEG09MYbJU0ReZbC+z5w5SZEDoKNz6x/9ScAdD/IRASC+ B7YUVundx+yO43YdFQDROl3qhC9n0Ujglc/hrATYDDfJ45hT36yxCE28UN69+3vzPtY1 UjD8lv5aoVFXhCQk2KZaViBGgNttAYntUv4uX1tsmpierqCDX2WKPeXZuy16UXpbKM1v o0VUhHAJAT1ETUqvKddcK3B9uo3/Dj25PlKOS8uyOaAnzg2W6KlawOyvVmR4G5Ue1zPR 350g== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20240605; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:dkim-signature; bh=pxOAFje5nPkMc5Q034nTfNvqywtXDIriDUwzfUf+f2Q=; fh=RNuMQWLuzL+4j5oo3RsTjQhIIRL6zkkPmJ9LnBb6B3k=; b=Byc4zPa1xIbnxjZePM+DRdVabrFuUf5BOexFpmx1zcxB1a4ciAP7QYGZ3vmludO6PL 3ZYmslv3n3jhkh+BBQcC2uMBAg2U/qeNhavgjQJPMKPWDQUx3nMMhExxQFLms3dy63N8 9R0ckuq6F2zuvwtaaa9bV5g2uho2CqORAkelhz3f0Z6wXyPtRiXfDiOYuY8ckqnqjpqI Iy4ZXmBNpgDypPTSadY2q4mbzlIEdnWihsDypHZzzZRIqdMzSHC7r/qfIR5FR/24VG2G W9vj206gu8yG1LVcUfqgkiopKuwlNC/3Sp8BVWA2c9YC+mhUmPrA9t6/A6dHK4cRYBwx dRwA==; darn=vger.kernel.org ARC-Authentication-Results: i=1; mx.google.com; arc=none DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1775635752; x=1776240552; darn=vger.kernel.org; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:from:to:cc:subject:date :message-id:reply-to; bh=pxOAFje5nPkMc5Q034nTfNvqywtXDIriDUwzfUf+f2Q=; b=M50qj+b/VDN8hZ4uXUuzUJGmnAcW3nY5ojog7V5YkhzFRvuq/vEvZ5Fa6+Srr5lXCK dpuzTXFz4zTLx7K76ASMFORVzCcBHou6IrPAG0gWaUUeA1FKiQnUIC3mHclYJQQ+eqn4 n6JbSIigLNEoy2v3L9PjYaGqUGey037CxLRNNHPS2+DOnmC5nMqqJ+l1ojBpq81PUsli WNC+gy6ltrVQeDnwfm8Ynw4neIR+IdAYaLyYQ532pOW5+8EE1jcp1Laq05QRnnqVg1pl T8WqqHnVxyPI3KyO5QHQDbQwrCtVx3A/zECh5t8sT5KRySO5CDqjMprmL7NdxWtO4bEy GYWA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1775635752; x=1776240552; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-gg:x-gm-message-state:from :to:cc:subject:date:message-id:reply-to; bh=pxOAFje5nPkMc5Q034nTfNvqywtXDIriDUwzfUf+f2Q=; b=A4mHqdApNBjjtmh0bvQtovS+NvXAGE/dk+94wtvuDA5MdgeIKNvVLVvqO2F+7oxmts H2XJqKUuasmaA+lVw52dIfMohVE/gNah9t0bY6WFViBaecnqgaZFCzTXuX3j07qLPWJS c4tA616wDqoOGJF45HEqCdGs9YsKXuvzHFFikBExDRBKRwgY5M4BMm/q7DAAQH41n0YM uNrUAVPpkDlyO0GK3AdqQ759G5nsr1ctdYQE8tN7QTuGnkRij70pBCTOG1Ad60p2Ws6m DrukhHvIJu2cxVa1QCTGHTqT8sGqCKMztE9J5BCbV4J2IMKu0NrsI8MclUCLg3Cynsoe 401A== X-Forwarded-Encrypted: i=1; AJvYcCVWrP3qXaUV28Y019wOzB27sCQrA24UzipX2fZDrSuIE+c34J8e3Sdim8HN/kIHkB8gC9sqtDU/GbCJmTM=@vger.kernel.org X-Gm-Message-State: AOJu0Yy+sKrdIhEwsQ62WaDALQJNGmQdtIkQdE+FB/txh2LTx9SPUCL+ VCUrKjvUq9LSCYBtlXFngxvlNcPAi2F+Ib2qSI/U76ZWzduz52vGghqA+SCSn8Y98pZYdXNXsvT Wk5YmjuQNjyXNzrzVETVDHPXiFMkeX9M= X-Gm-Gg: AeBDiesyXM4Kn3dYLUmzJ3yC9g0MPX5o+8M0DnKYDHVPXQVO4gco2t6cuOrOtbRDNox WN/JOHD5sxSY9nqcYXOlRspVjE4gSFQAv/gPhcktnVTKsS/4XplfHcb+st1LIkCNG0oYakvO3RZ mbTDPGYYrztGPvUO0iYdhgS2vc0c40rnrUbCw/yBT4IcpvhK/I7ljLnwLMM/r7EejhMPhwXJ60Y 4nRlDU6FnlpkIYARiAjo3bU1/xUCJ1LZLesUNAn6RkCK2ie7FgiTmaSGxrCD8JH38ett5Pma3wi v373SvQ= X-Received: by 2002:a05:651c:1b04:b0:38b:e048:572 with SMTP id 38308e7fff4ca-38d8d1dc57dmr59181801fa.0.1775635752037; Wed, 08 Apr 2026 01:09:12 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20260407081442.6256-1-komlomal@gmail.com> In-Reply-To: From: "Denis M. Karpov" Date: Wed, 8 Apr 2026 11:09:00 +0300 X-Gm-Features: AQROBzBgCH4RjVweEEM1TCje7jtGEZHThFkvR0yPukwiJ1tBezDT9yaTd4pJIDs Message-ID: Subject: Re: [RFC PATCH] userfaultfd: allow registration of ranges below mmap_min_addr To: "Harry Yoo (Oracle)" , Andrea Arcangeli Cc: 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable > Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory exte= rnalization") Thank you, I will add this Fixes tag in the next patch. > Hmm but it looks bit strange to check capability for address that is > already mapped by mmap(). Why is this required? Actually, it's not obvious to me either, but I may miss something. My inten= t was to replace the current restrictive check with a more flexible one. I think performing this check here allows us to deny invalid requests early, before locks or VMA lookups occur. Removing this check entirely would also allow using UFFD in cases where a t= ask drops privileges after the initial mmap(). This seems reasonable because th= e VMA already exists, i.e. kernel already allowed this mapping. In the [BUG] thread discussion Andrea Arcangeli also suggested adding a check for FIRST_USER_ADDRESS to handle architectural constraints. Andrea, could you please comment on this? Specifically, would a check against FIRST_USER_ADDRESS sufficient here, or do we still need to check caps? On Wed, Apr 8, 2026 at 6:21=E2=80=AFAM Harry Yoo (Oracle) wrote: > > On Tue, Apr 07, 2026 at 11:14:42AM +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 memo= ry > > regions mapped by application. > > > > Replace the rigid mmap_min_addr check with security_mmap_addr() to alig= n > > userfaultfd with the standard kernel memory mapping security policy. > > Perhaps worth adding > > Fixes: 86039bd3b4e6 ("userfaultfd: add new syscall to provide memory exte= rnalization") > > > Signed-off-by: Denis M. Karpov > > > > --- > > 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_r= ange( > > return -EINVAL; > > if (!len) > > return -EINVAL; > > - if (start < mmap_min_addr) > > - return -EINVAL; > > if (start >=3D task_size) > > return -EINVAL; > > if (len > task_size - start) > > return -EINVAL; > > if (start + len <=3D start) > > return -EINVAL; > > - return 0; > > + return security_mmap_addr(start); > > Hmm but it looks bit strange to check capability for address that is > already mapped by mmap(). Why is this required? > > > } > > -- > Cheers, > Harry / Hyeonggon