Re: [PATCH] mm: fix hugetlb bug due to user_shm_unlock call (fwd)
From: Hugh Dickins
Date: Mon Aug 24 2009 - 08:28:40 EST
On Fri, 23 Aug 2009, Stefan Huber wrote:
> We have recently detected a kernel oops bug in the hugetlb code. The bug
> is still present in the current linux-2.6 git branch (tested until [1]).
> We have attached a 'git format-patch'-file that solved problem for us.
> The commit message should describe the logic of the bug. Please contact
> me if you have further questions or comments.
>
> Sincerely,
> Stefan Huber.
>
> [1] linux/kernel/git/torvalds/linux-2.6.git (commit
> 429966b8f644dda2afddb4f834a944e9b46a7645)
That's a valuable discovery, thank you for reporting it.
However, though I can well believe that your patch works well for you,
I don't think it's general enough: there is no guarantee that the tests
in can_do_hugetlb_shm() will give the same answer to the user who ends
up calling shm_destroy() as it did once upon a time to the user who
called hugetlb_file_setup().
So, please could you try this alternative patch below, to see if it
passes your testing too, and let us know the result? I'm sure we'd
like to get a fix into 2.6.31, and into 2.6.30-stable.
Thanks,
Hugh
[PATCH] mm: fix hugetlb bug due to user_shm_unlock call
2.6.30's commit 8a0bdec194c21c8fdef840989d0d7b742bb5d4bc removed
user_shm_lock() calls in hugetlb_file_setup() but left the
user_shm_unlock call in shm_destroy().
In detail:
Assume that can_do_hugetlb_shm() returns true and hence user_shm_lock()
is not called in hugetlb_file_setup(). However, user_shm_unlock() is
called in any case in shm_destroy() and in the following
atomic_dec_and_lock(&up->__count) in free_uid() is executed and if
up->__count gets zero, also cleanup_user_struct() is scheduled.
Note that sched_destroy_user() is empty if CONFIG_USER_SCHED is not set.
However, the ref counter up->__count gets unexpectedly non-positive and
the corresponding structs are freed even though there are live
references to them, resulting in a kernel oops after a lots of
shmget(SHM_HUGETLB)/shmctl(IPC_RMID) cycles and CONFIG_USER_SCHED set.
Reported-by: Stefan Huber <shuber2@xxxxxxxxx>
Signed-off-by: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxx
---
fs/hugetlbfs/inode.c | 20 ++++++++++++--------
include/linux/hugetlb.h | 6 ++++--
ipc/shm.c | 6 +++---
3 files changed, 19 insertions(+), 13 deletions(-)
--- 2.6.31-rc7/fs/hugetlbfs/inode.c 2009-06-25 05:18:06.000000000 +0100
+++ linux/fs/hugetlbfs/inode.c 2009-08-24 12:32:01.000000000 +0100
@@ -935,26 +935,28 @@ static int can_do_hugetlb_shm(void)
return capable(CAP_IPC_LOCK) || in_group_p(sysctl_hugetlb_shm_group);
}
-struct file *hugetlb_file_setup(const char *name, size_t size, int acctflag)
+struct file *hugetlb_file_setup(const char *name, size_t size, int acctflag,
+ struct user_struct **user)
{
int error = -ENOMEM;
- int unlock_shm = 0;
struct file *file;
struct inode *inode;
struct dentry *dentry, *root;
struct qstr quick_string;
- struct user_struct *user = current_user();
+ *user = NULL;
if (!hugetlbfs_vfsmount)
return ERR_PTR(-ENOENT);
if (!can_do_hugetlb_shm()) {
- if (user_shm_lock(size, user)) {
- unlock_shm = 1;
+ *user = current_user();
+ if (user_shm_lock(size, *user)) {
WARN_ONCE(1,
"Using mlock ulimits for SHM_HUGETLB deprecated\n");
- } else
+ } else {
+ *user = NULL;
return ERR_PTR(-EPERM);
+ }
}
root = hugetlbfs_vfsmount->mnt_root;
@@ -996,8 +998,10 @@ out_inode:
out_dentry:
dput(dentry);
out_shm_unlock:
- if (unlock_shm)
- user_shm_unlock(size, user);
+ if (*user) {
+ user_shm_unlock(size, *user);
+ *user = NULL;
+ }
return ERR_PTR(error);
}
--- 2.6.31-rc7/include/linux/hugetlb.h 2009-06-25 05:18:08.000000000 +0100
+++ linux/include/linux/hugetlb.h 2009-08-24 12:32:01.000000000 +0100
@@ -10,6 +10,7 @@
#include <asm/tlbflush.h>
struct ctl_table;
+struct user_struct;
int PageHuge(struct page *page);
@@ -146,7 +147,8 @@ static inline struct hugetlbfs_sb_info *
extern const struct file_operations hugetlbfs_file_operations;
extern struct vm_operations_struct hugetlb_vm_ops;
-struct file *hugetlb_file_setup(const char *name, size_t, int);
+struct file *hugetlb_file_setup(const char *name, size_t size, int acct,
+ struct user_struct **user);
int hugetlb_get_quota(struct address_space *mapping, long delta);
void hugetlb_put_quota(struct address_space *mapping, long delta);
@@ -168,7 +170,7 @@ static inline void set_file_hugepages(st
#define is_file_hugepages(file) 0
#define set_file_hugepages(file) BUG()
-#define hugetlb_file_setup(name,size,acctflag) ERR_PTR(-ENOSYS)
+#define hugetlb_file_setup(name,size,acct,user) ERR_PTR(-ENOSYS)
#endif /* !CONFIG_HUGETLBFS */
--- 2.6.31-rc7/ipc/shm.c 2009-06-25 05:18:09.000000000 +0100
+++ linux/ipc/shm.c 2009-08-24 12:32:01.000000000 +0100
@@ -174,7 +174,7 @@ static void shm_destroy(struct ipc_names
shm_unlock(shp);
if (!is_file_hugepages(shp->shm_file))
shmem_lock(shp->shm_file, 0, shp->mlock_user);
- else
+ else if (shp->mlock_user)
user_shm_unlock(shp->shm_file->f_path.dentry->d_inode->i_size,
shp->mlock_user);
fput (shp->shm_file);
@@ -369,8 +369,8 @@ static int newseg(struct ipc_namespace *
/* hugetlb_file_setup applies strict accounting */
if (shmflg & SHM_NORESERVE)
acctflag = VM_NORESERVE;
- file = hugetlb_file_setup(name, size, acctflag);
- shp->mlock_user = current_user();
+ file = hugetlb_file_setup(name, size, acctflag,
+ &shp->mlock_user);
} else {
/*
* Do not allow no accounting for OVERCOMMIT_NEVER, even
--
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