[PATCH 0/2] Allow reading process DMA buf stats from fdinfo

[PATCH 0/2] Allow reading process DMA buf stats from fdinfo

[Kalesh Singh]

Android captures per-process system memory state when certain low memory
events (e.g a foreground app kill) occur, to identify potential memory
hoggers. In order to measure how much memory a process actually consumes,
it is necessary to include the DMA buffer sizes for that process in the
memory accounting. Since the handle to DMA buffers are raw FDs, it is
important to be able to identify which processes have FD references to
a DMA buffer.

Currently, DMA buffer FDs can be accounted using /proc/<pid>/fd/* and
/proc/<pid>/fdinfo -- both are only readable by the process owner,
as follows:
  1. Do a readlink on each FD.
  2. If the target path begins with "/dmabuf", then the FD is a dmabuf FD.
  3. stat the file to get the dmabuf inode number.
  4. Read/ proc/<pid>/fdinfo/<fd>, to get the DMA buffer size.

Accessing other processes’ fdinfo requires root privileges. This limits
the use of the interface to debugging environments and is not suitable
for production builds.  Granting root privileges even to a system process
increases the attack surface and is highly undesirable.

This series proposes making the requirement to read fdinfo less strict
with PTRACE_MODE_READ.


Kalesh Singh (2):
  procfs: Allow reading fdinfo with PTRACE_MODE_READ
  dmabuf: Add dmabuf inode no to fdinfo

 drivers/dma-buf/dma-buf.c |  1 +
 fs/proc/base.c            |  4 ++--
 fs/proc/fd.c              | 15 ++++++++++++++-
 3 files changed, 17 insertions(+), 3 deletions(-)

-- 
2.30.0.365.g02bc693789-goog

[PATCH 1/2] procfs: Allow reading fdinfo with PTRACE_MODE_READ

[Kalesh Singh]

Since fdinfo doesn't permit reading process memory and manipulating
process state, allow accessing fdinfo under PTRACE_MODE_READ_FSCRED.

Suggested-by: Jann Horn <jannh@google.com>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---
 fs/proc/base.c |  4 ++--
 fs/proc/fd.c   | 15 ++++++++++++++-
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index b3422cda2a91..a37f9de7103f 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3160,7 +3160,7 @@ static const struct pid_entry tgid_base_stuff[] = {
 	DIR("task",       S_IRUGO|S_IXUGO, proc_task_inode_operations, proc_task_operations),
 	DIR("fd",         S_IRUSR|S_IXUSR, proc_fd_inode_operations, proc_fd_operations),
 	DIR("map_files",  S_IRUSR|S_IXUSR, proc_map_files_inode_operations, proc_map_files_operations),
-	DIR("fdinfo",     S_IRUSR|S_IXUSR, proc_fdinfo_inode_operations, proc_fdinfo_operations),
+	DIR("fdinfo",     S_IRUGO|S_IXUGO, proc_fdinfo_inode_operations, proc_fdinfo_operations),
 	DIR("ns",	  S_IRUSR|S_IXUGO, proc_ns_dir_inode_operations, proc_ns_dir_operations),
 #ifdef CONFIG_NET
 	DIR("net",        S_IRUGO|S_IXUGO, proc_net_inode_operations, proc_net_operations),
@@ -3504,7 +3504,7 @@ static const struct inode_operations proc_tid_comm_inode_operations = {
  */
 static const struct pid_entry tid_base_stuff[] = {
 	DIR("fd",        S_IRUSR|S_IXUSR, proc_fd_inode_operations, proc_fd_operations),
-	DIR("fdinfo",    S_IRUSR|S_IXUSR, proc_fdinfo_inode_operations, proc_fdinfo_operations),
+	DIR("fdinfo",    S_IRUGO|S_IXUGO, proc_fdinfo_inode_operations, proc_fdinfo_operations),
 	DIR("ns",	 S_IRUSR|S_IXUGO, proc_ns_dir_inode_operations, proc_ns_dir_operations),
 #ifdef CONFIG_NET
 	DIR("net",        S_IRUGO|S_IXUGO, proc_net_inode_operations, proc_net_operations),
diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index cb51763ed554..585e213301f9 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -6,6 +6,7 @@
 #include <linux/fdtable.h>
 #include <linux/namei.h>
 #include <linux/pid.h>
+#include <linux/ptrace.h>
 #include <linux/security.h>
 #include <linux/file.h>
 #include <linux/seq_file.h>
@@ -72,6 +73,18 @@ static int seq_show(struct seq_file *m, void *v)
 
 static int seq_fdinfo_open(struct inode *inode, struct file *file)
 {
+	bool allowed = false;
+	struct task_struct *task = get_proc_task(inode);
+
+	if (!task)
+		return -ESRCH;
+
+	allowed = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
+	put_task_struct(task);
+
+	if (!allowed)
+		return -EACCES;
+
 	return single_open(file, seq_show, inode);
 }
 
@@ -307,7 +320,7 @@ static struct dentry *proc_fdinfo_instantiate(struct dentry *dentry,
 	struct proc_inode *ei;
 	struct inode *inode;
 
-	inode = proc_pid_make_inode(dentry->d_sb, task, S_IFREG | S_IRUSR);
+	inode = proc_pid_make_inode(dentry->d_sb, task, S_IFREG | S_IRUGO);
 	if (!inode)
 		return ERR_PTR(-ENOENT);
 
-- 
2.30.0.365.g02bc693789-goog

[PATCH 2/2] dmabuf: Add dmabuf inode no to fdinfo

[Kalesh Singh]

The dmabuf inode number allows userspace to uniquely identify the buffer
and avoids a dependency on /proc/<pid>/fd/* when accounting per-process
DMA buffer sizes.

Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
---
 drivers/dma-buf/dma-buf.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 9ad6397aaa97..d869099ede83 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -414,6 +414,7 @@ static void dma_buf_show_fdinfo(struct seq_file *m, struct file *file)
 {
 	struct dma_buf *dmabuf = file->private_data;
 
+	seq_printf(m, "dmabuf_inode_no:\t%lu\n", file_inode(file)->i_ino);
 	seq_printf(m, "size:\t%zu\n", dmabuf->size);
 	/* Don't count the temporary reference taken inside procfs seq_show */
 	seq_printf(m, "count:\t%ld\n", file_count(dmabuf->file) - 1);
-- 
2.30.0.365.g02bc693789-goog

Re: [PATCH 0/2] Allow reading process DMA buf stats from fdinfo

[Kalesh Singh]

On Thu, Jan 28, 2021 at 1:24 PM Kalesh Singh <kaleshsingh@google.com> wrote:
>
> Android captures per-process system memory state when certain low memory
> events (e.g a foreground app kill) occur, to identify potential memory
> hoggers. In order to measure how much memory a process actually consumes,
> it is necessary to include the DMA buffer sizes for that process in the
> memory accounting. Since the handle to DMA buffers are raw FDs, it is
> important to be able to identify which processes have FD references to
> a DMA buffer.
>
> Currently, DMA buffer FDs can be accounted using /proc/<pid>/fd/* and
> /proc/<pid>/fdinfo -- both are only readable by the process owner,
> as follows:
>   1. Do a readlink on each FD.
>   2. If the target path begins with "/dmabuf", then the FD is a dmabuf FD.
>   3. stat the file to get the dmabuf inode number.
>   4. Read/ proc/<pid>/fdinfo/<fd>, to get the DMA buffer size.
>
> Accessing other processes’ fdinfo requires root privileges. This limits
> the use of the interface to debugging environments and is not suitable
> for production builds.  Granting root privileges even to a system process
> increases the attack surface and is highly undesirable.
>
> This series proposes making the requirement to read fdinfo less strict
> with PTRACE_MODE_READ.
>

Hi everyone,

I will send v2 of this patch series. Please let me know if you have
any other comments or feedback, that should be addressed in the new
version.

Thanks,
Kalesh

> Kalesh Singh (2):
>   procfs: Allow reading fdinfo with PTRACE_MODE_READ
>   dmabuf: Add dmabuf inode no to fdinfo
>
>  drivers/dma-buf/dma-buf.c |  1 +
>  fs/proc/base.c            |  4 ++--
>  fs/proc/fd.c              | 15 ++++++++++++++-
>  3 files changed, 17 insertions(+), 3 deletions(-)
>
> --
> 2.30.0.365.g02bc693789-goog
>