RE: [PATCH] dmabuf: use spinlock to access dmabuf->name

[Ruhl, Michael J]

>-----Original Message-----
>From: dri-devel <dri-devel-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of
>Charan Teja Kalla
>Sent: Thursday, June 11, 2020 9:40 AM
>To: Sumit Semwal <sumit.semwal@xxxxxxxxxx>; open list:DMA BUFFER
>SHARING FRAMEWORK <linux-media@xxxxxxxxxxxxxxx>; DRI mailing list <dri-
>devel@xxxxxxxxxxxxxxxxxxxxx>
>Cc: Linaro MM SIG <linaro-mm-sig@xxxxxxxxxxxxxxxx>;
>vinmenon@xxxxxxxxxxxxxx; LKML <linux-kernel@xxxxxxxxxxxxxxx>;
>stable@xxxxxxxxxxxxxxx
>Subject: [PATCH] dmabuf: use spinlock to access dmabuf->name
>
>There exists a sleep-while-atomic bug while accessing the dmabuf->name
>under mutex in the dmabuffs_dname(). This is caused from the SELinux
>permissions checks on a process where it tries to validate the inherited
>files from fork() by traversing them through iterate_fd() (which
>traverse files under spin_lock) and call
>match_file(security/selinux/hooks.c) where the permission checks happen.
>This audit information is logged using dump_common_audit_data() where it
>calls d_path() to get the file path name. If the file check happen on
>the dmabuf's fd, then it ends up in ->dmabuffs_dname() and use mutex to
>access dmabuf->name. The flow will be like below:
>flush_unauthorized_files()
>  iterate_fd()
>    spin_lock() --> Start of the atomic section.
>      match_file()
>        file_has_perm()
>          avc_has_perm()
>            avc_audit()
>              slow_avc_audit()
>	        common_lsm_audit()
>		  dump_common_audit_data()
>		    audit_log_d_path()
>		      d_path()
>                        dmabuffs_dname()
>                          mutex_lock()--> Sleep while atomic.
>
>Call trace captured (on 4.19 kernels) is below:
>___might_sleep+0x204/0x208
>__might_sleep+0x50/0x88
>__mutex_lock_common+0x5c/0x1068
>__mutex_lock_common+0x5c/0x1068
>mutex_lock_nested+0x40/0x50
>dmabuffs_dname+0xa0/0x170
>d_path+0x84/0x290
>audit_log_d_path+0x74/0x130
>common_lsm_audit+0x334/0x6e8
>slow_avc_audit+0xb8/0xf8
>avc_has_perm+0x154/0x218
>file_has_perm+0x70/0x180
>match_file+0x60/0x78
>iterate_fd+0x128/0x168
>selinux_bprm_committing_creds+0x178/0x248
>security_bprm_committing_creds+0x30/0x48
>install_exec_creds+0x1c/0x68
>load_elf_binary+0x3a4/0x14e0
>search_binary_handler+0xb0/0x1e0
>
>So, use spinlock to access dmabuf->name to avoid sleep-while-atomic.
>
>Cc: <stable@xxxxxxxxxxxxxxx> [5.3+]
>Signed-off-by: Charan Teja Reddy <charante@xxxxxxxxxxxxxx>
>---
> drivers/dma-buf/dma-buf.c | 13 +++++++------
> include/linux/dma-buf.h   |  1 +
> 2 files changed, 8 insertions(+), 6 deletions(-)
>
>diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>index 01ce125..2e0456c 100644
>--- a/drivers/dma-buf/dma-buf.c
>+++ b/drivers/dma-buf/dma-buf.c
>@@ -45,10 +45,10 @@ static char *dmabuffs_dname(struct dentry *dentry,
>char *buffer, int buflen)
> 	size_t ret = 0;
>
> 	dmabuf = dentry->d_fsdata;
>-	dma_resv_lock(dmabuf->resv, NULL);
>+	spin_lock(&dmabuf->name_lock);
> 	if (dmabuf->name)
> 		ret = strlcpy(name, dmabuf->name, DMA_BUF_NAME_LEN);
>-	dma_resv_unlock(dmabuf->resv);
>+	spin_unlock(&dmabuf->name_lock);

I am not really clear on why you need this lock.

If name == NULL you have no issues.
If name is real, you have no issues.

If name is freed you will copy garbage, but the only way
for that to happen is that _set_name or _release have to be called
at just the right time.

And the above would probably only be an issue if the set_name
was called, so you will get NULL or a real name.

Is there a reason for the lock here?

Mike

> 	return dynamic_dname(dentry, buffer, buflen, "/%s:%s",
> 			     dentry->d_name.name, ret > 0 ? name : "");
>@@ -335,7 +335,7 @@ static long dma_buf_set_name(struct dma_buf
>*dmabuf, const char __user *buf)
> 	if (IS_ERR(name))
> 		return PTR_ERR(name);
>
>-	dma_resv_lock(dmabuf->resv, NULL);
>+	spin_lock(&dmabuf->name_lock);
> 	if (!list_empty(&dmabuf->attachments)) {
> 		ret = -EBUSY;
> 		kfree(name);
>@@ -345,7 +345,7 @@ static long dma_buf_set_name(struct dma_buf
>*dmabuf, const char __user *buf)
> 	dmabuf->name = name;
>
> out_unlock:
>-	dma_resv_unlock(dmabuf->resv);
>+	spin_unlock(&dmabuf->name_lock);
> 	return ret;
> }
>
>@@ -405,10 +405,10 @@ static void dma_buf_show_fdinfo(struct seq_file
>*m, struct file *file)
> 	/* Don't count the temporary reference taken inside procfs seq_show
>*/
> 	seq_printf(m, "count:\t%ld\n", file_count(dmabuf->file) - 1);
> 	seq_printf(m, "exp_name:\t%s\n", dmabuf->exp_name);
>-	dma_resv_lock(dmabuf->resv, NULL);
>+	spin_lock(&dmabuf->name_lock);
> 	if (dmabuf->name)
> 		seq_printf(m, "name:\t%s\n", dmabuf->name);
>-	dma_resv_unlock(dmabuf->resv);
>+	spin_unlock(&dmabuf->name_lock);
> }
>
> static const struct file_operations dma_buf_fops = {
>@@ -546,6 +546,7 @@ struct dma_buf *dma_buf_export(const struct
>dma_buf_export_info *exp_info)
> 	dmabuf->size = exp_info->size;
> 	dmabuf->exp_name = exp_info->exp_name;
> 	dmabuf->owner = exp_info->owner;
>+	spin_lock_init(&dmabuf->name_lock);
> 	init_waitqueue_head(&dmabuf->poll);
> 	dmabuf->cb_excl.poll = dmabuf->cb_shared.poll = &dmabuf->poll;
> 	dmabuf->cb_excl.active = dmabuf->cb_shared.active = 0;
>diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
>index ab0c156..93108fd 100644
>--- a/include/linux/dma-buf.h
>+++ b/include/linux/dma-buf.h
>@@ -311,6 +311,7 @@ struct dma_buf {
> 	void *vmap_ptr;
> 	const char *exp_name;
> 	const char *name;
>+	spinlock_t name_lock;
> 	struct module *owner;
> 	struct list_head list_node;
> 	void *priv;
>--
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