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fs: fix data races on inode->i_flctx
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locks_get_lock_context() uses cmpxchg() to install i_flctx.
cmpxchg() is a release operation which is correct. But it uses
a plain load to load i_flctx. This is incorrect. Subsequent loads
from i_flctx can hoist above the load of i_flctx pointer itself
and observe uninitialized garbage there. This in turn can lead
to corruption of ctx->flc_lock and other members.

Documentation/memory-barriers.txt explicitly requires to use
a barrier in such context:
"A load-load control dependency requires a full read memory barrier".

Use smp_load_acquire() in locks_get_lock_context() and in bunch
of other functions that can proceed concurrently with
locks_get_lock_context().

The data race was found with KernelThreadSanitizer (KTSAN).

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
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Dmitry Vyukov authored and Jeff Layton committed Sep 21, 2015
1 parent d11797a commit 128a378
Showing 1 changed file with 36 additions and 27 deletions.
63 changes: 36 additions & 27 deletions fs/locks.c
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Original file line number Diff line number Diff line change
Expand Up @@ -205,28 +205,32 @@ static struct kmem_cache *filelock_cache __read_mostly;
static struct file_lock_context *
locks_get_lock_context(struct inode *inode, int type)
{
struct file_lock_context *new;
struct file_lock_context *ctx;

if (likely(inode->i_flctx) || type == F_UNLCK)
/* paired with cmpxchg() below */
ctx = smp_load_acquire(&inode->i_flctx);
if (likely(ctx) || type == F_UNLCK)
goto out;

new = kmem_cache_alloc(flctx_cache, GFP_KERNEL);
if (!new)
ctx = kmem_cache_alloc(flctx_cache, GFP_KERNEL);
if (!ctx)
goto out;

spin_lock_init(&new->flc_lock);
INIT_LIST_HEAD(&new->flc_flock);
INIT_LIST_HEAD(&new->flc_posix);
INIT_LIST_HEAD(&new->flc_lease);
spin_lock_init(&ctx->flc_lock);
INIT_LIST_HEAD(&ctx->flc_flock);
INIT_LIST_HEAD(&ctx->flc_posix);
INIT_LIST_HEAD(&ctx->flc_lease);

/*
* Assign the pointer if it's not already assigned. If it is, then
* free the context we just allocated.
*/
if (cmpxchg(&inode->i_flctx, NULL, new))
kmem_cache_free(flctx_cache, new);
if (cmpxchg(&inode->i_flctx, NULL, ctx)) {
kmem_cache_free(flctx_cache, ctx);
ctx = smp_load_acquire(&inode->i_flctx);
}
out:
return inode->i_flctx;
return ctx;
}

void
Expand Down Expand Up @@ -762,7 +766,7 @@ posix_test_lock(struct file *filp, struct file_lock *fl)
struct file_lock_context *ctx;
struct inode *inode = file_inode(filp);

ctx = inode->i_flctx;
ctx = smp_load_acquire(&inode->i_flctx);
if (!ctx || list_empty_careful(&ctx->flc_posix)) {
fl->fl_type = F_UNLCK;
return;
Expand Down Expand Up @@ -1203,7 +1207,7 @@ int locks_mandatory_locked(struct file *file)
struct file_lock_context *ctx;
struct file_lock *fl;

ctx = inode->i_flctx;
ctx = smp_load_acquire(&inode->i_flctx);
if (!ctx || list_empty_careful(&ctx->flc_posix))
return 0;

Expand Down Expand Up @@ -1388,7 +1392,7 @@ any_leases_conflict(struct inode *inode, struct file_lock *breaker)
int __break_lease(struct inode *inode, unsigned int mode, unsigned int type)
{
int error = 0;
struct file_lock_context *ctx = inode->i_flctx;
struct file_lock_context *ctx;
struct file_lock *new_fl, *fl, *tmp;
unsigned long break_time;
int want_write = (mode & O_ACCMODE) != O_RDONLY;
Expand All @@ -1400,6 +1404,7 @@ int __break_lease(struct inode *inode, unsigned int mode, unsigned int type)
new_fl->fl_flags = type;

/* typically we will check that ctx is non-NULL before calling */
ctx = smp_load_acquire(&inode->i_flctx);
if (!ctx) {
WARN_ON_ONCE(1);
return error;
Expand Down Expand Up @@ -1494,9 +1499,10 @@ EXPORT_SYMBOL(__break_lease);
void lease_get_mtime(struct inode *inode, struct timespec *time)
{
bool has_lease = false;
struct file_lock_context *ctx = inode->i_flctx;
struct file_lock_context *ctx;
struct file_lock *fl;

ctx = smp_load_acquire(&inode->i_flctx);
if (ctx && !list_empty_careful(&ctx->flc_lease)) {
spin_lock(&ctx->flc_lock);
if (!list_empty(&ctx->flc_lease)) {
Expand Down Expand Up @@ -1543,10 +1549,11 @@ int fcntl_getlease(struct file *filp)
{
struct file_lock *fl;
struct inode *inode = file_inode(filp);
struct file_lock_context *ctx = inode->i_flctx;
struct file_lock_context *ctx;
int type = F_UNLCK;
LIST_HEAD(dispose);

ctx = smp_load_acquire(&inode->i_flctx);
if (ctx && !list_empty_careful(&ctx->flc_lease)) {
spin_lock(&ctx->flc_lock);
time_out_leases(file_inode(filp), &dispose);
Expand Down Expand Up @@ -1713,9 +1720,10 @@ static int generic_delete_lease(struct file *filp, void *owner)
struct file_lock *fl, *victim = NULL;
struct dentry *dentry = filp->f_path.dentry;
struct inode *inode = dentry->d_inode;
struct file_lock_context *ctx = inode->i_flctx;
struct file_lock_context *ctx;
LIST_HEAD(dispose);

ctx = smp_load_acquire(&inode->i_flctx);
if (!ctx) {
trace_generic_delete_lease(inode, NULL);
return error;
Expand Down Expand Up @@ -2359,13 +2367,14 @@ int fcntl_setlk64(unsigned int fd, struct file *filp, unsigned int cmd,
void locks_remove_posix(struct file *filp, fl_owner_t owner)
{
struct file_lock lock;
struct file_lock_context *ctx = file_inode(filp)->i_flctx;
struct file_lock_context *ctx;

/*
* If there are no locks held on this file, we don't need to call
* posix_lock_file(). Another process could be setting a lock on this
* file at the same time, but we wouldn't remove that lock anyway.
*/
ctx = smp_load_acquire(&file_inode(filp)->i_flctx);
if (!ctx || list_empty(&ctx->flc_posix))
return;

Expand All @@ -2389,7 +2398,7 @@ EXPORT_SYMBOL(locks_remove_posix);

/* The i_flctx must be valid when calling into here */
static void
locks_remove_flock(struct file *filp)
locks_remove_flock(struct file *filp, struct file_lock_context *flctx)
{
struct file_lock fl = {
.fl_owner = filp,
Expand All @@ -2400,7 +2409,6 @@ locks_remove_flock(struct file *filp)
.fl_end = OFFSET_MAX,
};
struct inode *inode = file_inode(filp);
struct file_lock_context *flctx = inode->i_flctx;

if (list_empty(&flctx->flc_flock))
return;
Expand All @@ -2416,10 +2424,8 @@ locks_remove_flock(struct file *filp)

/* The i_flctx must be valid when calling into here */
static void
locks_remove_lease(struct file *filp)
locks_remove_lease(struct file *filp, struct file_lock_context *ctx)
{
struct inode *inode = file_inode(filp);
struct file_lock_context *ctx = inode->i_flctx;
struct file_lock *fl, *tmp;
LIST_HEAD(dispose);

Expand All @@ -2439,17 +2445,20 @@ locks_remove_lease(struct file *filp)
*/
void locks_remove_file(struct file *filp)
{
if (!file_inode(filp)->i_flctx)
struct file_lock_context *ctx;

ctx = smp_load_acquire(&file_inode(filp)->i_flctx);
if (!ctx)
return;

/* remove any OFD locks */
locks_remove_posix(filp, filp);

/* remove flock locks */
locks_remove_flock(filp);
locks_remove_flock(filp, ctx);

/* remove any leases */
locks_remove_lease(filp);
locks_remove_lease(filp, ctx);
}

/**
Expand Down Expand Up @@ -2616,7 +2625,7 @@ void show_fd_locks(struct seq_file *f,
struct file_lock_context *ctx;
int id = 0;

ctx = inode->i_flctx;
ctx = smp_load_acquire(&inode->i_flctx);
if (!ctx)
return;

Expand Down

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