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SUNRPC: Track writers of the 'channel' file to improve cache_listeners_exist 

The sunrpc cache interface is susceptible to being fooled by a rogue
process just reading a 'channel' file.  If this happens the kernel
may think a valid daemon exists to service the cache when it does not.
For example, the following may fool the kernel:
cat /proc/net/rpc/auth.unix.gid/channel

Change the tracking of readers to writers when considering whether a
listener exists as all valid daemon processes either open a channel
file O_RDWR or O_WRONLY.  While this does not prevent a rogue process
from "stealing" a message from the kernel, it does at least improve
the kernels perception of whether a valid process servicing the cache
exists.

Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
This commit is contained in:
Dave Wysochanski 2019-07-26 18:33:01 -04:00 committed by J. Bruce Fields
parent 609488bc97
commit 64a38e840c
2 changed files with 11 additions and 7 deletions
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6
include/linux/sunrpc/cache.h
View File

@ -107,9 +107,9 @@ struct cache_detail {
/* fields for communication over channel */
struct list_head queue;
atomic_t readers; /* how many time is /chennel open */
time_t last_close; /* if no readers, when did last close */
time_t last_warn; /* when we last warned about no readers */
atomic_t writers; /* how many time is /channel open */
time_t last_close; /* if no writers, when did last close */
time_t last_warn; /* when we last warned about no writers */
union {
struct proc_dir_entry *procfs;

12
net/sunrpc/cache.c
View File

@ -373,7 +373,7 @@ void sunrpc_init_cache_detail(struct cache_detail *cd)
spin_lock(&cache_list_lock);
cd->nextcheck = 0;
cd->entries = 0;
atomic_set(&cd->readers, 0);
atomic_set(&cd->writers, 0);
cd->last_close = 0;
cd->last_warn = -1;
list_add(&cd->others, &cache_list);
@ -1029,11 +1029,13 @@ static int cache_open(struct inode *inode, struct file *filp,
}
rp->offset = 0;
rp->q.reader = 1;
atomic_inc(&cd->readers);
spin_lock(&queue_lock);
list_add(&rp->q.list, &cd->queue);
spin_unlock(&queue_lock);
}
if (filp->f_mode & FMODE_WRITE)
atomic_inc(&cd->writers);
filp->private_data = rp;
return 0;
}
@ -1062,8 +1064,10 @@ static int cache_release(struct inode *inode, struct file *filp,
filp->private_data = NULL;
kfree(rp);
}
if (filp->f_mode & FMODE_WRITE) {
atomic_dec(&cd->writers);
cd->last_close = seconds_since_boot();
atomic_dec(&cd->readers);
}
module_put(cd->owner);
return 0;
@ -1171,7 +1175,7 @@ static void warn_no_listener(struct cache_detail *detail)
static bool cache_listeners_exist(struct cache_detail *detail)
{
if (atomic_read(&detail->readers))
if (atomic_read(&detail->writers))
return true;
if (detail->last_close == 0)
/* This cache was never opened */