Re: linux-next: Tree for Apr 12

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On Thu, 12 Apr 2012 14:24:15 -0700
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Thu, 12 Apr 2012 14:59:31 +1000
> Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx> wrote:
> 
> > I have created today's linux-next tree at
> > git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git
> 
> This isn't working for me.  Some time between April 3 and April 12
> someone merged something into the non-mm part of linux-next which broke
> ssh.
> 
> I boot the box and everything seems to come up OK, but attemtps to ssh
> into the machine fail with
> 
> X11 forwarding request failed on channel 0
> Last login: Thu Apr 12 13:04:35 2012 from akpm.corp.google.com
> Connection to akpm2 closed.
> 
> I took a peek in the `strace ssh' output.
> 
> Good:
> 
> 17815 write(5, "Last login: Thu Apr 12 13:27:23 "..., 65) = 65
> 17815 select(7, [3 4], [], NULL, {120, 0}) = 1 (in [3], left {119, 770798})
> 17815 read(3, "\21O\200\366Mv\343\222\332\251\2403L\376Y18\2047\336\244\226p-+X\2%\2119\314\255"..., 8192) = 80
> 17815 select(7, [3 4], [5], NULL, {120, 0}) = 1 (out [5], left {119, 999987})
> 17815 write(5, "\r\33[m\17\33[27m\33[24m\33[Jakpm2:/home/ak"..., 39) = 39
> 17815 select(7, [3 4], [], NULL, {120, 0}) = 1 (in [4], left {118, 801111})
> 17815 read(4, "\4", 16384)              = 1
> 17815 select(7, [3 4], [3], NULL, {120, 0}) = 1 (out [3], left {119, 999991})
> 17815 write(3, "\235J\5\340\234\21\266\207\26e\362\327\2\332\1\267\272\200\364\267?/\320L\341\35\350{+M:\222"..., 48) = 48
> 
> 
> Bad:
> 
> 9305  write(5, "Last login: Thu Apr 12 13:02:54 "..., 65) = 65
> 9305  select(7, [3 4], [], NULL, {120, 0}) = 1 (in [3], left {119, 945541})
> 9305  read(3, "f\357\250~\260i\2259\320\3258\262)O\364;_\251\360-\314\31\374]\326\300\356\364\370S\3105"..., 8192) = 128
> 9305  close(5)                          = 0
> 9305  close(4)                          = 0
> 
> That read() is returning a lot more data.
> 
> It appears that we've done something which breaks X forwarding.  I
> won't be able to look any further into this until Monday.

This regression is now in mainline.  I've bisected it to an SELinux
patch, below.  I have confirmed that reverting just that patch from
current mainline fixes the regression.

Using openssh-server-4.3p2-14.fc6 on FC6.


commit 95dbf739313f09c8d859bde1373bc264ef979337
Author:     Eric Paris <eparis@xxxxxxxxxx>
AuthorDate: Wed Apr 4 13:45:34 2012 -0400
Commit:     Eric Paris <eparis@xxxxxxxxxx>
CommitDate: Mon Apr 9 12:22:49 2012 -0400

    SELinux: check OPEN on truncate calls
    
    In RH BZ 578841 we realized that the SELinux sandbox program was allowed to
    truncate files outside of the sandbox.  The reason is because sandbox
    confinement is determined almost entirely by the 'open' permission.  The idea
    was that if the sandbox was unable to open() files it would be unable to do
    harm to those files.  This turns out to be false in light of syscalls like
    truncate() and chmod() which don't require a previous open() call.  I looked
    at the syscalls that did not have an associated 'open' check and found that
    truncate(), did not have a seperate permission and even if it did have a
    separate permission such a permission owuld be inadequate for use by
    sandbox (since it owuld have to be granted so liberally as to be useless).
    This patch checks the OPEN permission on truncate.  I think a better solution
    for sandbox is a whole new permission, but at least this fixes what we have
    today.
    
    Signed-off-by: Eric Paris <eparis@xxxxxxxxxx>

diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index d85b793..f7d7e77 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2708,6 +2708,7 @@ static int selinux_inode_setattr(struct dentry *dentry, struct iattr *iattr)
 {
 	const struct cred *cred = current_cred();
 	unsigned int ia_valid = iattr->ia_valid;
+	__u32 av = FILE__WRITE;
 
 	/* ATTR_FORCE is just used for ATTR_KILL_S[UG]ID. */
 	if (ia_valid & ATTR_FORCE) {
@@ -2721,7 +2722,10 @@ static int selinux_inode_setattr(struct dentry *dentry, struct iattr *iattr)
 			ATTR_ATIME_SET | ATTR_MTIME_SET | ATTR_TIMES_SET))
 		return dentry_has_perm(cred, dentry, FILE__SETATTR);
 
-	return dentry_has_perm(cred, dentry, FILE__WRITE);
+	if (ia_valid & ATTR_SIZE)
+		av |= FILE__OPEN;
+
+	return dentry_has_perm(cred, dentry, av);
 }
 
 static int selinux_inode_getattr(struct vfsmount *mnt, struct dentry *dentry)

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