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b=C8mqCAa3sOez8bYlcdk7Ex9hl1Zc3wGzNlS+gMfhYDFWo2GO6U1oOyWzlb2bPQY+x hqp45+zj52D2OrfKyI0wE6OnSs4tXIB0pDV9h5BA+FBwqNLLssYqEG2iralfKBbWze 8TnH2nZgoDMCRUVPIIfWgQ5CX2TqVmWguLZl5ry0GMyg4/zCAi/p/yVuCqOjBPDh1S 3CfPkRN86lNOdPWBsocKmexkv/xlcBg6n1fCMbp7GWE8C9TU8+qYac8CviSfGdpNx7 2tBQ+HKflUJA/Bs03oR8x3vLAb4l3YfkvzAafZcN8aqTCzekcrr22Gj5IiA+WkyfGs NrtPDLo03P3hg== Date: Fri, 31 Jul 2026 11:14:08 -0700 From: Eric Biggers To: Baoquan He Cc: linux-mm@kvack.org, Andrew Morton , Chris Li , Kairui Song , linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-fscrypt@vger.kernel.org, linux-kernel@vger.kernel.org, Kemeng Shi , Nhat Pham , Barry Song , Youngjun Park , stable@vger.kernel.org Subject: Re: [PATCH] mm/swap: reject swapon() on filesystem-level encrypted files Message-ID: <20260731181408.GA12803@sol> References: <20260730184853.48347-1-ebiggers@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Stat-Signature: os7zyqyyog98pw59x1e76onqt64fwmf3 X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: B1E72A000E X-HE-Tag: 1785521764-562880 X-HE-Meta: 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 jqK2aPiT FVc6pBleZyr6BOXQkgYNOypGVdueRB27CKlL/50FasHHVd8Q1em0YgTwh8Lm4eNU7p8cShBCEVkvhuw5PTQNmeFQ4D6R8Lf2PSwQv2DZsZldMlyMhdjkN2mdtYsnOahBzQxFjE6EKqdlb3fokNLJGRjMW7+jMJDdQAIiMdClGG8RirBSevIU8N2gnyAgPn7uTdUGfF2Vy+JHAGmGD/RP6B3qZUw25FRIUym9QhQ1w9B3BDe9Mnwl1tz79YK+EgAMHKS+1n0uZ33KJDyk+8y9UR5SX7g== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Jul 31, 2026 at 11:11:02PM +0800, Baoquan He wrote: > On 07/30/26 at 11:48am, Eric Biggers wrote: > > ext4 and f2fs don't prevent filesystem-level encrypted files from being > > set up directly as swap files. In this case, encryption is bypassed. > > > > No one should be doing this, vs. the methods of encrypted swap that > > actually do work (such as swapping to a dm-crypt device, or swapping to > > a loopback device on top of a filesystem-level encrypted file). > > > > Nevertheless, to prevent user error, make swapon() explicitly reject > > this case. Document this behavior in fscrypt.rst as well. > > > > Fixes: 9bd8212f981e ("ext4 crypto: add encryption policy and password salt support") > > Fixes: f424f664f0e8 ("f2fs crypto: add encryption policy and password salt support") > > Cc: stable@vger.kernel.org > > Signed-off-by: Eric Biggers > > --- > > Documentation/filesystems/fscrypt.rst | 4 ++++ > > mm/swapfile.c | 4 ++++ > > 2 files changed, 8 insertions(+) > > > > diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst > > index c0dd35f1af12..cba1989777da 100644 > > --- a/Documentation/filesystems/fscrypt.rst > > +++ b/Documentation/filesystems/fscrypt.rst > > @@ -1238,6 +1238,10 @@ astute users may notice some differences in behavior: > > > > - DAX (Direct Access) is not supported on encrypted files. > > > > +- Encrypted files cannot be used directly as swap files. To swap to > > + an encrypted file, set up a loopback device on top of it. > > + Alternatively, encrypted swap can use a dm-crypt device instead. > > + > > - The maximum length of an encrypted symlink is 2 bytes shorter than > > the maximum length of an unencrypted symlink. For example, on an > > EXT4 filesystem with a 4K block size, unencrypted symlinks can be up > > diff --git a/mm/swapfile.c b/mm/swapfile.c > > index 78b49b0658ad..e4991da81b5f 100644 > > --- a/mm/swapfile.c > > +++ b/mm/swapfile.c > > @@ -3650,6 +3650,10 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags) > > error = -EBUSY; > > goto bad_swap_unlock_inode; > > } > > + if (IS_ENCRYPTED(inode)) { > > + error = -EINVAL; > > + goto bad_swap_unlock_inode; > > + } > > Sounds reasonable to me. > > One minor suggestion: it might be helpful to emit a pr_warn_once() > explaining *why* the swapon was rejected and pointing users to the > dm-crypt / loopback alternatives, so the -EINVAL isn't opaque. > Optional though -- the check itself is what matters. How's this? pr_warn_once("Filesystem-level encrypted swapfile '%s' is unsupported. Create a loop device over it, or use dm-crypt\n", name->name); > And this patch updates fscrypt.rst. Would it be good to also > update the swapon(8) man page to document the new -EINVAL > condition? > > Other than these, > > Reviewed-by: Baoquan He swapon(8) is for the util-linux program, not the syscall, so it doesn't document error codes specifically. It does have a NOTES section that incompletely documents filesystem-specific constraints on swap files. It's missing most of the current constraints. But sure, I'll send a patch that adds a note to there about this one. That will be a util-linux patch. - Eric