From: Kalesh Singh <kaleshsingh@xxxxxxxxxx> Subject: procfs: allow reading fdinfo with PTRACE_MODE_READ 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. Link: https://lkml.kernel.org/r/20210308170651.919148-1-kaleshsingh@xxxxxxxxxx Signed-off-by: Kalesh Singh <kaleshsingh@xxxxxxxxxx> Suggested-by: Jann Horn <jannh@xxxxxxxxxx> Acked-by: Christian König <christian.koenig@xxxxxxx> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx> Cc: Alexey Gladkov <gladkov.alexey@xxxxxxxxx> Cc: Andrei Vagin <avagin@xxxxxxxxx> Cc: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx> Cc: Christian Brauner <christian.brauner@xxxxxxxxxx> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx> Cc: Helge Deller <deller@xxxxxx> Cc: Hridya Valsaraju <hridya@xxxxxxxxxx> Cc: James Morris <jamorris@xxxxxxxxxxxxxxxxxxx> Cc: Jeff Vander Stoep <jeffv@xxxxxxxxxx> Cc: Jonathan Corbet <corbet@xxxxxxx> Cc: Kees Cook <keescook@xxxxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Mauro Carvalho Chehab <mchehab+huawei@xxxxxxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxx> Cc: Michel Lespinasse <walken@xxxxxxxxxx> Cc: Minchan Kim <minchan@xxxxxxxxxx> Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx> Cc: Szabolcs Nagy <szabolcs.nagy@xxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/proc/base.c | 4 ++-- fs/proc/fd.c | 15 ++++++++++++++- 2 files changed, 16 insertions(+), 3 deletions(-) --- a/fs/proc/base.c~procfs-allow-reading-fdinfo-with-ptrace_mode_read +++ a/fs/proc/base.c @@ -3172,7 +3172,7 @@ static const struct pid_entry tgid_base_ 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), @@ -3517,7 +3517,7 @@ static const struct inode_operations pro */ 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), --- a/fs/proc/fd.c~procfs-allow-reading-fdinfo-with-ptrace_mode_read +++ a/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 @@ out: 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_instan 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); _
