Re: [RESEND PATCH v6 1/2] procfs: Allow reading fdinfo with PTRACE_MODE_READ

[Christian König]

Am 08.03.21 um 18:06 schrieb 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.
>
> 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@xxxxxxxxxx>
> Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx>

Both patches are Acked-by: Christian König <christian.koenig@xxxxxxx>

> ---
> Hi everyone,
>
> The initial posting of this patch can be found at [1].
> I didn't receive any feedback last time, so resending here.
> Would really appreciate any constructive comments/suggestions.
>
> Thanks,
> Kalesh
>
> [1] https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fr%2F20210208155315.1367371-1-kaleshsingh%40google.com%2F&amp;data=04%7C01%7Cchristian.koenig%40amd.com%7C38c98420f0564e15117f08d8e2549ff5%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637508200431130855%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&amp;sdata=deJBlAk6%2BEQkfAC8iRK95xhV1%2FiO9Si%2Bylc5Z0QzzrM%3D&amp;reserved=0
>
> Changes in v2:
>    - Update patch description
>   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 3851bfcdba56..fd46d8dd0cf4 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3159,7 +3159,7 @@ 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 07fc4fad2602..6a80b40fd2fe 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);
>   }
>   
> @@ -308,7 +321,7 @@ 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);
>