2020-03-04 10:59:32

by Cengiz Can

Subject: [PATCH v2] blktrace: fix dereference after null check

There was a recent change in blktrace.c that added a RCU protection to
`q->blk_trace` in order to fix a use-after-free issue during access.

However the change missed an edge case that can lead to dereferencing of
`bt` pointer even when it's NULL:

Coverity static analyzer marked this as a FORWARD_NULL issue with CID
1460458.

```
/kernel/trace/blktrace.c: 1904 in sysfs_blk_trace_attr_store()
1898 ret = 0;
1899 if (bt == NULL)
1900 ret = blk_trace_setup_queue(q, bdev);
1901
1902 if (ret == 0) {
1903 if (attr == &dev_attr_act_mask)
>>> CID 1460458: Null pointer dereferences (FORWARD_NULL)
>>> Dereferencing null pointer "bt".
1904 bt->act_mask = value;
1905 else if (attr == &dev_attr_pid)
1906 bt->pid = value;
1907 else if (attr == &dev_attr_start_lba)
1908 bt->start_lba = value;
1909 else if (attr == &dev_attr_end_lba)
```

Added a reassignment with RCU annotation to fix the issue.

Fixes: c780e86dd48 ("blktrace: Protect q->blk_trace with RCU")

Signed-off-by: Cengiz Can <[email protected]>
---

Patch Changelog
* v2: Added RCU annotation to assignment

kernel/trace/blktrace.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index 4560878f0bac..ca39dc3230cb 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -1896,8 +1896,11 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev,
}

ret = 0;
- if (bt == NULL)
+ if (bt == NULL) {
ret = blk_trace_setup_queue(q, bdev);
+ bt = rcu_dereference_protected(q->blk_trace,
+ lockdep_is_held(&q->blk_trace_mutex));
+ }

if (ret == 0) {
if (attr == &dev_attr_act_mask)
--
2.25.1

2020-03-05 08:49:35

by Ming Lei

Subject: Re: [PATCH v2] blktrace: fix dereference after null check

On Wed, Mar 4, 2020 at 6:59 PM Cengiz Can <[email protected]> wrote:
>
> There was a recent change in blktrace.c that added a RCU protection to
> `q->blk_trace` in order to fix a use-after-free issue during access.
>
> However the change missed an edge case that can lead to dereferencing of
> `bt` pointer even when it's NULL:
>
> Coverity static analyzer marked this as a FORWARD_NULL issue with CID
> 1460458.
>
> ```
> /kernel/trace/blktrace.c: 1904 in sysfs_blk_trace_attr_store()
> 1898 ret = 0;
> 1899 if (bt == NULL)
> 1900 ret = blk_trace_setup_queue(q, bdev);
> 1901
> 1902 if (ret == 0) {
> 1903 if (attr == &dev_attr_act_mask)
> >>> CID 1460458: Null pointer dereferences (FORWARD_NULL)
> >>> Dereferencing null pointer "bt".
> 1904 bt->act_mask = value;
> 1905 else if (attr == &dev_attr_pid)
> 1906 bt->pid = value;
> 1907 else if (attr == &dev_attr_start_lba)
> 1908 bt->start_lba = value;
> 1909 else if (attr == &dev_attr_end_lba)
> ```
>
> Added a reassignment with RCU annotation to fix the issue.
>
> Fixes: c780e86dd48 ("blktrace: Protect q->blk_trace with RCU")
>
> Signed-off-by: Cengiz Can <[email protected]>
> ---
>
> Patch Changelog
> * v2: Added RCU annotation to assignment
>
> kernel/trace/blktrace.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index 4560878f0bac..ca39dc3230cb 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -1896,8 +1896,11 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev,
> }
>
> ret = 0;
> - if (bt == NULL)
> + if (bt == NULL) {
> ret = blk_trace_setup_queue(q, bdev);
> + bt = rcu_dereference_protected(q->blk_trace,
> + lockdep_is_held(&q->blk_trace_mutex));
> + }
>
> if (ret == 0) {
> if (attr == &dev_attr_act_mask)
> --
> 2.25.1
>

Reviewed-by: Ming Lei <[email protected]>

--
Ming Lei

2020-03-05 10:16:16

by Bob Liu

Subject: Re: [PATCH v2] blktrace: fix dereference after null check

On 3/4/20 6:58 PM, Cengiz Can wrote:
> There was a recent change in blktrace.c that added a RCU protection to
> `q->blk_trace` in order to fix a use-after-free issue during access.
>
> However the change missed an edge case that can lead to dereferencing of
> `bt` pointer even when it's NULL:
>
> Coverity static analyzer marked this as a FORWARD_NULL issue with CID
> 1460458.
>
> ```
> /kernel/trace/blktrace.c: 1904 in sysfs_blk_trace_attr_store()
> 1898 ret = 0;
> 1899 if (bt == NULL)
> 1900 ret = blk_trace_setup_queue(q, bdev);
> 1901
> 1902 if (ret == 0) {
> 1903 if (attr == &dev_attr_act_mask)
>>>> CID 1460458: Null pointer dereferences (FORWARD_NULL)
>>>> Dereferencing null pointer "bt".
> 1904 bt->act_mask = value;
> 1905 else if (attr == &dev_attr_pid)
> 1906 bt->pid = value;
> 1907 else if (attr == &dev_attr_start_lba)
> 1908 bt->start_lba = value;
> 1909 else if (attr == &dev_attr_end_lba)
> ```
>
> Added a reassignment with RCU annotation to fix the issue.
>
> Fixes: c780e86dd48 ("blktrace: Protect q->blk_trace with RCU")
>
> Signed-off-by: Cengiz Can <[email protected]>

This version looks good to me, thanks.
Reviewed-by: Bob Liu <[email protected]>

> ---
>
> Patch Changelog
> * v2: Added RCU annotation to assignment
>
> kernel/trace/blktrace.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index 4560878f0bac..ca39dc3230cb 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -1896,8 +1896,11 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev,
> }
>
> ret = 0;
> - if (bt == NULL)
> + if (bt == NULL) {
> ret = blk_trace_setup_queue(q, bdev);
> + bt = rcu_dereference_protected(q->blk_trace,
> + lockdep_is_held(&q->blk_trace_mutex));
> + }
>
> if (ret == 0) {
> if (attr == &dev_attr_act_mask)
> --
> 2.25.1
>

2020-03-05 14:08:21

by Steven Rostedt

Subject: Re: [PATCH v2] blktrace: fix dereference after null check

On Wed, 4 Mar 2020 13:58:19 +0300
Cengiz Can <[email protected]> wrote:

> There was a recent change in blktrace.c that added a RCU protection to
> `q->blk_trace` in order to fix a use-after-free issue during access.
>
> However the change missed an edge case that can lead to dereferencing of
> `bt` pointer even when it's NULL:
>
> Coverity static analyzer marked this as a FORWARD_NULL issue with CID
> 1460458.
>
> ```
> /kernel/trace/blktrace.c: 1904 in sysfs_blk_trace_attr_store()
> 1898 ret = 0;
> 1899 if (bt == NULL)
> 1900 ret = blk_trace_setup_queue(q, bdev);
> 1901
> 1902 if (ret == 0) {
> 1903 if (attr == &dev_attr_act_mask)
> >>> CID 1460458: Null pointer dereferences (FORWARD_NULL)
> >>> Dereferencing null pointer "bt".
> 1904 bt->act_mask = value;
> 1905 else if (attr == &dev_attr_pid)
> 1906 bt->pid = value;
> 1907 else if (attr == &dev_attr_start_lba)
> 1908 bt->start_lba = value;
> 1909 else if (attr == &dev_attr_end_lba)
> ```
>
> Added a reassignment with RCU annotation to fix the issue.
>
> Fixes: c780e86dd48 ("blktrace: Protect q->blk_trace with RCU")
>
> Signed-off-by: Cengiz Can <[email protected]>


Reviewed-by: Steven Rostedt (VMware) <[email protected]>

-- Steve

> ---
>
> Patch Changelog
> * v2: Added RCU annotation to assignment
>
> kernel/trace/blktrace.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index 4560878f0bac..ca39dc3230cb 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -1896,8 +1896,11 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev,
> }
>
> ret = 0;
> - if (bt == NULL)
> + if (bt == NULL) {
> ret = blk_trace_setup_queue(q, bdev);
> + bt = rcu_dereference_protected(q->blk_trace,
> + lockdep_is_held(&q->blk_trace_mutex));
> + }
>
> if (ret == 0) {
> if (attr == &dev_attr_act_mask)
> --
> 2.25.1

2020-03-05 20:44:11

by Jens Axboe

Subject: Re: [PATCH v2] blktrace: fix dereference after null check

On 3/4/20 3:58 AM, Cengiz Can wrote:
> There was a recent change in blktrace.c that added a RCU protection to
> `q->blk_trace` in order to fix a use-after-free issue during access.
>
> However the change missed an edge case that can lead to dereferencing of
> `bt` pointer even when it's NULL:
>
> Coverity static analyzer marked this as a FORWARD_NULL issue with CID
> 1460458.
>
> ```
> /kernel/trace/blktrace.c: 1904 in sysfs_blk_trace_attr_store()
> 1898 ret = 0;
> 1899 if (bt == NULL)
> 1900 ret = blk_trace_setup_queue(q, bdev);
> 1901
> 1902 if (ret == 0) {
> 1903 if (attr == &dev_attr_act_mask)
>>>> CID 1460458: Null pointer dereferences (FORWARD_NULL)
>>>> Dereferencing null pointer "bt".
> 1904 bt->act_mask = value;
> 1905 else if (attr == &dev_attr_pid)
> 1906 bt->pid = value;
> 1907 else if (attr == &dev_attr_start_lba)
> 1908 bt->start_lba = value;
> 1909 else if (attr == &dev_attr_end_lba)
> ```
>
> Added a reassignment with RCU annotation to fix the issue.

Applied, thanks.

--
Jens Axboe