> On Jul 7, 2016, at 13:07, Oleg Drokin <green@xxxxxxxxxxxxxx> wrote: > > > On Jul 7, 2016, at 12:59 PM, Trond Myklebust wrote: > >> >>> On Jul 7, 2016, at 12:52, Oleg Drokin <green@xxxxxxxxxxxxxx> wrote: >>> >>> >>> On Jul 7, 2016, at 12:16 PM, Trond Myklebust wrote: >>> >>>> >>>>> On Jul 7, 2016, at 01:53, Oleg Drokin <green@xxxxxxxxxxxxxx> wrote: >>>>> >>>>> It's great when we can shave an extra RPC, but not at the expense >>>>> of correctness. >>>>> We should not return EPERM (from vfs_create/mknod/mkdir) if the >>>>> name already exists, even if we have no write access in parent. >>>>> >>>>> Since the check in nfs_permission is clearly not enough to stave >>>>> off this, just throw in the extra READ access to actually >>>>> go through. >>>>> >>>>> Signed-off-by: Oleg Drokin <green@xxxxxxxxxxxxxx> >>>>> --- >>>>> fs/nfs/dir.c | 4 +++- >>>>> 1 file changed, 3 insertions(+), 1 deletion(-) >>>>> >>>>> diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c >>>>> index d8015a0..8c7835b 100644 >>>>> --- a/fs/nfs/dir.c >>>>> +++ b/fs/nfs/dir.c >>>>> @@ -1383,8 +1383,10 @@ struct dentry *nfs_lookup(struct inode *dir, struct dentry * dentry, unsigned in >>>>> /* >>>>> * If we're doing an exclusive create, optimize away the lookup >>>>> * but don't hash the dentry. >>>>> + * This optimization only works if we can write in the parent. >>>>> */ >>>>> - if (nfs_is_exclusive_create(dir, flags)) >>>>> + if (nfs_is_exclusive_create(dir, flags) && >>>>> + (inode_permission(dir, MAY_WRITE | MAY_READ | MAY_EXEC) == 0)) >>>>> return NULL; >>>>> >>>> >>>> NACK. The only write permission we should care about on the client side is whether or not the filesystem is mounted read-only. All other permissions are checked by the server. >>> >>> Right. This was mostly a discussion piece. >>> The problem here is nfs_permission() returns 0 if you check for >>> inode_permission(dir, MAY_WRITE | MAY_EXEC) (as in may_create), but then >>> some other checks in the kernel still catch on to the fact that the directory >>> is not writeable, so we have a premature failure with EPERM and server never sees >>> this request which breaks things. >> >> Are these VFS level checks? Which ones? > > Yes, VFS level I believe. > For Lustre it's may_create() from vfs_mkdir() that stops us short > and the Lustre patch is effective. > but for NFS this must be something else and I did not trace > it completely. One of the security checks, I guess? > if NFS patch is changed to check inode_permission(dir, MAY_OPEN | MAY_EXEC) > as in Lustre, that returns 0 no matter what. > > This is trivial to reproduce too. So, should we be ignoring the MAY_EXEC flag when the VFS asks for inode_permission(dir, MAY_WRITE|MAY_EXEC)? I suspect that is the problem here. > >>> (the read-only mount is not handled as well at the moment of course and my patch >>> does not address this issue either, but it's easier to address in the VFS, like >>> in filename_create() or something). >>> >>> I see that two major consumers of this nfs_permission MAY_WRITE|!MAY_READ check >>> are creates and deletes and with deletes we had a lookup already, so it already >>> looked up the child and revalidated the parent. >>> For creates, a revalidation still might be needed, I guess and that was the main driver >>> behind this check? And that only when you do current dir creates, because otherwise >>> the parent would have been revalidated in lookup? >>> Is this the major case why that check is actually there? >>> >>> Just trying to see how to approach this better without breaking the applications. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
