[PATCH 3.16 246/247] futex: Fix inode life-time issue

Ben Hutchings
2020-04-24T15:49:39Z

3.16.83-rc2 review patch. If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@infradead.org>

commit 8019ad13ef7f64be44d4f892af9c840179009254 upstream.

As reported by Jann, ihold() does not in fact guarantee inode
persistence. And instead of making it so, replace the usage of inode
pointers with a per boot, machine wide, unique inode identifier.

This sequence number is global, but shared (file backed) futexes are
rare enough that this should not become a performance issue.

Reported-by: Jann Horn <jannh@google.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.16: Use atomic64_cmpxchg() instead of the
_relaxed() variant which we don't have]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/inode.c | 1 +
include/linux/fs.h | 1 +
include/linux/futex.h | 17 +++++----
kernel/futex.c | 89 ++++++++++++++++++++++++++-----------------
4 files changed, 65 insertions(+), 43 deletions(-)

--- a/fs/inode.c
+++ b/fs/inode.c
@@ -131,6 +131,7 @@ int inode_init_always(struct super_block
inode->i_sb = sb;
inode->i_blkbits = sb->s_blocksize_bits;
inode->i_flags = 0;
+ atomic64_set(&inode->i_sequence, 0);
atomic_set(&inode->i_count, 1);
inode->i_op = &empty_iops;
inode->i_fop = &empty_fops;
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -570,6 +570,7 @@ struct inode {
struct rcu_head i_rcu;
};
u64 i_version;
+ atomic64_t i_sequence; /* see futex */
atomic_t i_count;
atomic_t i_dio_count;
atomic_t i_writecount;
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -39,23 +39,26 @@ handle_futex_death(u32 __user *uaddr, st

union futex_key {
struct {
+ u64 i_seq;
unsigned long pgoff;
- struct inode *inode;
- int offset;
+ unsigned int offset;
} shared;
struct {
+ union {
+ struct mm_struct *mm;
+ u64 __tmp;
+ };
unsigned long address;
- struct mm_struct *mm;
- int offset;
+ unsigned int offset;
} private;
struct {
+ u64 ptr;
unsigned long word;
- void *ptr;
- int offset;
+ unsigned int offset;
} both;
};

-#define FUTEX_KEY_INIT (union futex_key) { .both = { .ptr = NULL } }
+#define FUTEX_KEY_INIT (union futex_key) { .both = { .ptr = 0ULL } }

#ifdef CONFIG_FUTEX
extern void exit_robust_list(struct task_struct *curr);
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -338,7 +338,7 @@ static void get_futex_key_refs(union fut

switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
case FUT_OFF_INODE:
- ihold(key->shared.inode); /* implies MB (B) */
+ smp_mb(); /* explicit smp_mb(); (B) */
break;
case FUT_OFF_MMSHARED:
futex_get_mm(key); /* implies MB (B) */
@@ -362,7 +362,6 @@ static void drop_futex_key_refs(union fu

switch (key->both.offset & (FUT_OFF_INODE|FUT_OFF_MMSHARED)) {
case FUT_OFF_INODE:
- iput(key->shared.inode);
break;
case FUT_OFF_MMSHARED:
mmdrop(key->private.mm);
@@ -370,6 +369,46 @@ static void drop_futex_key_refs(union fu
}
}

+/*
+ * Generate a machine wide unique identifier for this inode.
+ *
+ * This relies on u64 not wrapping in the life-time of the machine; which with
+ * 1ns resolution means almost 585 years.
+ *
+ * This further relies on the fact that a well formed program will not unmap
+ * the file while it has a (shared) futex waiting on it. This mapping will have
+ * a file reference which pins the mount and inode.
+ *
+ * If for some reason an inode gets evicted and read back in again, it will get
+ * a new sequence number and will _NOT_ match, even though it is the exact same
+ * file.
+ *
+ * It is important that match_futex() will never have a false-positive, esp.
+ * for PI futexes that can mess up the state. The above argues that false-negatives
+ * are only possible for malformed programs.
+ */
+static u64 get_inode_sequence_number(struct inode *inode)
+{
+ static atomic64_t i_seq;
+ u64 old;
+
+ /* Does the inode already have a sequence number? */
+ old = atomic64_read(&inode->i_sequence);
+ if (likely(old))
+ return old;
+
+ for (;;) {
+ u64 new = atomic64_add_return(1, &i_seq);
+ if (WARN_ON_ONCE(!new))
+ continue;
+
+ old = atomic64_cmpxchg(&inode->i_sequence, 0, new);
+ if (old)
+ return old;
+ return new;
+ }
+}
+
/**
* get_futex_key() - Get parameters which are the keys for a futex
* @uaddr: virtual address of the futex
@@ -382,9 +421,15 @@ static void drop_futex_key_refs(union fu
*
* The key words are stored in *key on success.
*
- * For shared mappings, it's (page->index, file_inode(vma->vm_file),
- * offset_within_page). For private mappings, it's (uaddr, current->mm).
- * We can usually work out the index without swapping in the page.
+ * For shared mappings (when @fshared), the key is:
+ * ( inode->i_sequence, page->index, offset_within_page )
+ * [ also see get_inode_sequence_number() ]
+ *
+ * For private mappings (or when !@fshared), the key is:
+ * ( current->mm, address, 0 )
+ *
+ * This allows (cross process, where applicable) identification of the futex
+ * without keeping the page pinned for the duration of the FUTEX_WAIT.
*
* lock_page() might sleep, the caller should not hold a spinlock.
*/
@@ -545,8 +590,6 @@ again:
key->private.mm = mm;
key->private.address = address;

- get_futex_key_refs(key); /* implies smp_mb(); (B) */
-
} else {
struct inode *inode;

@@ -578,40 +621,14 @@ again:
goto again;
}

- /*
- * Take a reference unless it is about to be freed. Previously
- * this reference was taken by ihold under the page lock
- * pinning the inode in place so i_lock was unnecessary. The
- * only way for this check to fail is if the inode was
- * truncated in parallel which is almost certainly an
- * application bug. In such a case, just retry.
- *
- * We are not calling into get_futex_key_refs() in file-backed
- * cases, therefore a successful atomic_inc return below will
- * guarantee that get_futex_key() will still imply smp_mb(); (B).
- */
- if (!atomic_inc_not_zero(&inode->i_count)) {
- rcu_read_unlock();
- put_page(page_head);
-
- goto again;
- }
-
- /* Should be impossible but lets be paranoid for now */
- if (WARN_ON_ONCE(inode->i_mapping != mapping)) {
- err = -EFAULT;
- rcu_read_unlock();
- iput(inode);
-
- goto out;
- }
-
key->both.offset |= FUT_OFF_INODE; /* inode-based key */
- key->shared.inode = inode;
+ key->shared.i_seq = get_inode_sequence_number(inode);
key->shared.pgoff = basepage_index(page);
rcu_read_unlock();
}

+ get_futex_key_refs(key); /* implies smp_mb(); (B) */
+
out:
put_page(page_head);
return err;