-----Original Message----- From: charante=codeaurora.org@mg.codeaurora.org charante=codeaurora.org@mg.codeaurora.org On Behalf Of Charan Teja Kalla Sent: Tuesday, June 23, 2020 7:34 AM To: Ruhl, Michael J michael.j.ruhl@intel.com; Sumit Semwal sumit.semwal@linaro.org; David.Laight@ACULAB.COM; open list:DMA BUFFER SHARING FRAMEWORK linux-media@vger.kernel.org; DRI mailing list dri-devel@lists.freedesktop.org Cc: Linaro MM SIG linaro-mm-sig@lists.linaro.org; LKML <linux- kernel@vger.kernel.org> Subject: Re: [PATCH v2] dmabuf: use spinlock to access dmabuf->name
Thanks Mike for the inputs.
On 6/22/2020 5:10 PM, Ruhl, Michael J wrote:
-----Original Message----- From: charante=codeaurora.org@mg.codeaurora.org charante=codeaurora.org@mg.codeaurora.org On Behalf Of Charan
Teja
Kalla Sent: Monday, June 22, 2020 5:26 AM To: Ruhl, Michael J michael.j.ruhl@intel.com; Sumit Semwal sumit.semwal@linaro.org; David.Laight@ACULAB.COM; open list:DMA BUFFER SHARING FRAMEWORK linux-media@vger.kernel.org; DRI
mailing
list dri-devel@lists.freedesktop.org Cc: Linaro MM SIG linaro-mm-sig@lists.linaro.org; LKML <linux- kernel@vger.kernel.org> Subject: Re: [PATCH v2] dmabuf: use spinlock to access dmabuf->name
Hello Mike,
On 6/19/2020 7:11 PM, Ruhl, Michael J wrote:
-----Original Message----- From: charante=codeaurora.org@mg.codeaurora.org charante=codeaurora.org@mg.codeaurora.org On Behalf Of Charan
Teja
Kalla Sent: Friday, June 19, 2020 7:57 AM To: Sumit Semwal sumit.semwal@linaro.org; Ruhl, Michael J michael.j.ruhl@intel.com; David.Laight@ACULAB.COM; open
list:DMA
BUFFER SHARING FRAMEWORK linux-media@vger.kernel.org; DRI
mailing
list dri-devel@lists.freedesktop.org Cc: Linaro MM SIG linaro-mm-sig@lists.linaro.org; LKML <linux- kernel@vger.kernel.org> Subject: [PATCH v2] 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@vger.kernel.org [5.3+] Signed-off-by: Charan Teja Reddy charante@codeaurora.org
Changes in V2: Addressed review comments from Ruhl, Michael J
Changes in V1: https://lore.kernel.org/patchwork/patch/1255055/
drivers/dma-buf/dma-buf.c | 11 +++++++---- include/linux/dma-buf.h | 1 + 2 files changed, 8 insertions(+), 4 deletions(-)
diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index 01ce125..d81d298 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);
return dynamic_dname(dentry, buffer, buflen, "/%s:%s", dentry->d_name.name, ret > 0 ? name : "");
@@ -341,8 +341,10 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf) kfree(name); goto out_unlock; }
- spin_lock(&dmabuf->name_lock); kfree(dmabuf->name); dmabuf->name = name;
- spin_unlock(&dmabuf->name_lock);
While this code path is ok, I would have separated the protection of the attachment list and the name manipulation.
dma_resv_lock(resv) if (!list_empty(attachment) ret = -EBUSY dma_resv_unlock(resv)
if (ret) { kfree(name) return ret; }
Is it that the name should be visible before importer attaches to the dmabuf,(using dma_buf_attach()), thus _buf_set_name() is under the _resv_lock() as well?
That is the name that was being freed in the error path of the lock block. Alternatively:
dma_resv_lock(resv) if (!list_empty(attachment) { ret = -EBUSY kfree(name)
We can free this buffer even outside of the lock with out any issues. This is just a user passed name copied into local buffer yet to assign to dmabuf->name.
} dma_resv_unlock(resv)
if (ret) return ret;
I was limiting what was happening in the lock block.
You have two distinct locks, that protect two distinct items:
dmabuf->attachment dmabuf->name
Nesting the locking is ok, but if the code ever changes you can get that nesting wrong, so:
Your suggestion below looks clean, but what I am still not sure is that is there any condition like "there should be no attachments to the exported dmabuf before assigning the name" -- If yes, then _resv_lock and name_lock should be nested while assigning the name which otherwise breaks under below scenario:
I missed that piece of info, and I now understand the constraint.
Sorry for the extended conversation. 😊
I good with version 2.
Mike
P1 P2
buf_set_name() called and no attachments to this dmabuf yet. attaches to the exported dmabuf by P1.
Say it tries to get the name with the assumption that name is already set.Now it tries to change the name under just name_lock
In the above case P2 didn't get any name of the exported dmabuf despite name is set.
If not, then I can give V3 with the suggested changes..
long ret = 0;
if (IS_ERR(name)) return PTR_ERR(name);
dma_resv_lock(dmabuf->resv, NULL); if (!list_empty(&dmabuf->attachments)) { ret = -EBUSY; kfree(name); } dma_resv_unlock(dmabuf->resv); if (ret) return ret;
spinlock(dmabuf->name_lock) kfree(dmabuf->name); dmabuf->name = name; spinunlock(dmabuf->name_lock)
return 0; }
M
spinlock(nam_lock) ...
Nesting locks that don't need to be nested always makes me nervous for future use that misses the lock/unlock pattern.
However, this looks reasonable.
With this current code, or if you update to the above pattern:
Reviewed-by: Michael J. Ruhl michael.j.ruhl@intel.com
Thanks for the ACK.
Mike
out_unlock: dma_resv_unlock(dmabuf->resv); @@ -405,10 +407,10 @@ 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 +548,7 @@ 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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-- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project
-- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project