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

[Ruhl, Michael J]

>-----Original Message-----
>From: dri-devel <dri-devel-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of
>Ruhl, Michael J
>Sent: Tuesday, June 16, 2020 9:51 AM
>To: Charan Teja Kalla <charante@xxxxxxxxxxxxxx>; 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: RE: [PATCH] dmabuf: use spinlock to access dmabuf->name
>
>>-----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

Maybe dmabuf->name = NULL after the kfree(dmabuf->name) in:

dma_buf_release()

Would be sufficient?

M
>> 	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;
>>--
>>The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
>>Forum, a Linux Foundation Collaborative Project
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