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Subject: Re: [PATCH] Abort file_remove_privs() for non-reg. files
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     Jan Kara <jack@suse.cz>, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Horst Schirmeier <horst.schirmeier@tu-dortmund.de>
References: <4903939e-d3d6-b0c2-9c33-0fea0a61213c@tu-dortmund.de>
 <20181207175811.GZ2217@ZenIV.linux.org.uk>
 <5c86e85f-0ad4-935a-3021-7046551f361f@tu-dortmund.de>
 <20181208004944.GA2217@ZenIV.linux.org.uk>
 <20181210094722.GB29289@quack2.suse.cz>
 <cbdc8071-de76-bb0a-6890-15ef21023a70@tu-dortmund.de>
 <20181217082800.GA28270@quack2.suse.cz>
From:   Alexander Lochmann <alexander.lochmann@tu-dortmund.de>
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Message-ID: <3273d690-488f-b13c-5988-f600c4f83192@tu-dortmund.de>
Date:   Fri, 11 Jan 2019 16:42:10 +0100
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From: Alexander Lochmann <alexander.lochmann@tu-dortmund.de>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jan Kara <jack@suse.cz>, linux-fsdevel@vger.kernel.org,
 linux-kernel@vger.kernel.org,
 Horst Schirmeier <horst.schirmeier@tu-dortmund.de>
Message-ID: <3273d690-488f-b13c-5988-f600c4f83192@tu-dortmund.de>
Subject: Re: [PATCH] Abort file_remove_privs() for non-reg. files
References: <4903939e-d3d6-b0c2-9c33-0fea0a61213c@tu-dortmund.de>
 <20181207175811.GZ2217@ZenIV.linux.org.uk>
 <5c86e85f-0ad4-935a-3021-7046551f361f@tu-dortmund.de>
 <20181208004944.GA2217@ZenIV.linux.org.uk>
 <20181210094722.GB29289@quack2.suse.cz>
 <cbdc8071-de76-bb0a-6890-15ef21023a70@tu-dortmund.de>
 <20181217082800.GA28270@quack2.suse.cz>
In-Reply-To: <20181217082800.GA28270@quack2.suse.cz>

--TlRJm0tWjRuRlS33CMF3UBS9azF0r1pMn
Content-Type: text/plain; charset=utf-8
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Content-Transfer-Encoding: quoted-printable

Hello Al,

Have you had the opportunity to review our patch?

Cheers,
Alex

On 17.12.18 09:28, Jan Kara wrote:
> On Fri 14-12-18 11:55:52, Alexander Lochmann wrote:
>>
>> file_remove_privs() might be called for non-regular files, e.g.
>> blkdev inode. There is no reason to do its job on things
>> like blkdev inodes, pipes, or cdevs. Hence, abort if
>> file does not refer to a regular inode.
>> The following stacktrace shows how to get there:
>> 13: entry_SYSENTER_32:460
>> 12: do_fast_syscall_32:410
>> 11: _static_cpu_has:146
>> 10: do_syscall_32_irqs_on:322
>> 09: SyS_pwrite64:636
>> 08: SYSC_pwrite64:650
>> 07: fdput:38
>> 06: vfs_write:560
>> 05: __vfs_write:512
>> 04: new_sync_write:500
>> 03: blkdev_write_iter:1977
>> 02: __generic_file_write_iter:2897
>> 01: file_remove_privs:1818
>> 00: inode_has_no_xattr:3163
>>
>> Found by LockDoc (Alexander Lochmann, Horst Schirmeier and Olaf
>> Spinczyk)
>>
>> Signed-off-by: Alexander Lochmann <alexander.lochmann@tu-dortmund.de>
>> Signed-off-by: Horst Schirmeier <horst.schirmeier@tu-dortmund.de>
>=20
> The patch looks good to me. You can add:
>=20
> Reviewed-by: Jan Kara <jack@suse.cz>
>=20
> 								Honza
>=20
>> ---
>>  fs/inode.c | 9 +++++++--
>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/inode.c b/fs/inode.c
>> index 35d2108d567c..682088190413 100644
>> --- a/fs/inode.c
>> +++ b/fs/inode.c
>> @@ -1820,8 +1820,13 @@ int file_remove_privs(struct file *file)
>>  	int kill;
>>  	int error =3D 0;
>>
>> -	/* Fast path for nothing security related */
>> -	if (IS_NOSEC(inode))
>> +	/*
>> +	 * Fast path for nothing security related.
>> +	 * As well for non-regular files, e.g. blkdev inodes.
>> +	 * For example, blkdev_write_iter() might get here
>> +	 * trying to remove privs which it is not allowed to.
>> +	 */
>> +	if (IS_NOSEC(inode) || !S_ISREG(inode->i_mode))
>>  		return 0;
>>
>>  	kill =3D dentry_needs_remove_privs(dentry);
>> --=20
>> 2.19.2
>>
>=20
>=20
>=20

--=20
Technische Universit=C3=A4t Dortmund
Alexander Lochmann                PGP key: 0xBC3EF6FD
Otto-Hahn-Str. 16                 phone:  +49.231.7556141
D-44227 Dortmund                  fax:    +49.231.7556116
http://ess.cs.tu-dortmund.de/Staff/al


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