[PATCH 2/2] proc: protect ptrace_may_access() with exec_update_lock (FD links)
From: Jann Horn
Date: Mon May 18 2026 - 12:36:46 EST
proc_pid_get_link() and proc_pid_readlink() currently look up the task from
the pid once, then do the ptrace access check on that task, then look up
the task from the pid a second time to do the actual access.
That's racy in several ways.
To fix it, pass the task to the ->proc_get_link() handler, and instead of
proc_fd_access_allowed(), introduce a new helper call_proc_get_link() that
looks up and locks the task, does the access check, and calls
->proc_get_link().
Fixes: 778c1144771f ("[PATCH] proc: Use sane permission checks on the /proc/<pid>/fd/ symlinks")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---
fs/proc/base.c | 119 +++++++++++++++++++++--------------------------------
fs/proc/fd.c | 27 +++++-------
fs/proc/internal.h | 2 +-
3 files changed, 59 insertions(+), 89 deletions(-)
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 09b02d1621e5..ef2f59461374 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -218,33 +218,24 @@ static int get_task_root(struct task_struct *task, struct path *root)
return result;
}
-static int proc_cwd_link(struct dentry *dentry, struct path *path)
+static int proc_cwd_link(struct dentry *dentry, struct path *path,
+ struct task_struct *task)
{
- struct task_struct *task = get_proc_task(d_inode(dentry));
int result = -ENOENT;
- if (task) {
- task_lock(task);
- if (task->fs) {
- get_fs_pwd(task->fs, path);
- result = 0;
- }
- task_unlock(task);
- put_task_struct(task);
+ task_lock(task);
+ if (task->fs) {
+ get_fs_pwd(task->fs, path);
+ result = 0;
}
+ task_unlock(task);
return result;
}
-static int proc_root_link(struct dentry *dentry, struct path *path)
+static int proc_root_link(struct dentry *dentry, struct path *path,
+ struct task_struct *task)
{
- struct task_struct *task = get_proc_task(d_inode(dentry));
- int result = -ENOENT;
-
- if (task) {
- result = get_task_root(task, path);
- put_task_struct(task);
- }
- return result;
+ return get_task_root(task, path);
}
/*
@@ -710,23 +701,6 @@ static int proc_pid_syscall(struct seq_file *m, struct pid_namespace *ns,
/* Here the fs part begins */
/************************************************************************/
-/* permission checks */
-static bool proc_fd_access_allowed(struct inode *inode)
-{
- struct task_struct *task;
- bool allowed = false;
- /* Allow access to a task's file descriptors if it is us or we
- * may use ptrace attach to the process and find out that
- * information.
- */
- task = get_proc_task(inode);
- if (task) {
- allowed = ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS);
- put_task_struct(task);
- }
- return allowed;
-}
-
int proc_nochmod_setattr(struct mnt_idmap *idmap, struct dentry *dentry,
struct iattr *attr)
{
@@ -1783,16 +1757,12 @@ static const struct file_operations proc_pid_set_comm_operations = {
.release = single_release,
};
-static int proc_exe_link(struct dentry *dentry, struct path *exe_path)
+static int proc_exe_link(struct dentry *dentry, struct path *exe_path,
+ struct task_struct *task)
{
- struct task_struct *task;
struct file *exe_file;
- task = get_proc_task(d_inode(dentry));
- if (!task)
- return -ENOENT;
exe_file = get_task_exe_file(task);
- put_task_struct(task);
if (exe_file) {
*exe_path = exe_file->f_path;
path_get(&exe_file->f_path);
@@ -1802,26 +1772,42 @@ static int proc_exe_link(struct dentry *dentry, struct path *exe_path)
return -ENOENT;
}
+static int call_proc_get_link(struct dentry *dentry, struct inode *inode, struct path *path_out)
+{
+ struct task_struct *task;
+ int ret;
+
+ task = get_proc_task(inode);
+ if (!task)
+ return -ENOENT;
+ ret = down_read_killable(&task->signal->exec_update_lock);
+ if (ret)
+ goto out_put_task;
+ if (!ptrace_may_access(task, PTRACE_MODE_READ_FSCREDS)) {
+ ret = -EACCES;
+ goto out;
+ }
+ ret = PROC_I(inode)->op.proc_get_link(dentry, path_out, task);
+
+out:
+ up_read(&task->signal->exec_update_lock);
+out_put_task:
+ put_task_struct(task);
+ return ret;
+}
+
static const char *proc_pid_get_link(struct dentry *dentry,
struct inode *inode,
struct delayed_call *done)
{
struct path path;
- int error = -EACCES;
+ int error;
if (!dentry)
return ERR_PTR(-ECHILD);
-
- /* Are we allowed to snoop on the tasks file descriptors? */
- if (!proc_fd_access_allowed(inode))
- goto out;
-
- error = PROC_I(inode)->op.proc_get_link(dentry, &path);
- if (error)
- goto out;
-
- error = nd_jump_link(&path);
-out:
+ error = call_proc_get_link(dentry, inode, &path);
+ if (!error)
+ error = nd_jump_link(&path);
return ERR_PTR(error);
}
@@ -1855,17 +1841,11 @@ static int proc_pid_readlink(struct dentry * dentry, char __user * buffer, int b
struct inode *inode = d_inode(dentry);
struct path path;
- /* Are we allowed to snoop on the tasks file descriptors? */
- if (!proc_fd_access_allowed(inode))
- goto out;
-
- error = PROC_I(inode)->op.proc_get_link(dentry, &path);
- if (error)
- goto out;
-
- error = do_proc_readlink(&path, buffer, buflen);
- path_put(&path);
-out:
+ error = call_proc_get_link(dentry, inode, &path);
+ if (!error) {
+ error = do_proc_readlink(&path, buffer, buflen);
+ path_put(&path);
+ }
return error;
}
@@ -2256,21 +2236,16 @@ static const struct dentry_operations tid_map_files_dentry_operations = {
.d_delete = pid_delete_dentry,
};
-static int map_files_get_link(struct dentry *dentry, struct path *path)
+static int map_files_get_link(struct dentry *dentry, struct path *path,
+ struct task_struct *task)
{
unsigned long vm_start, vm_end;
struct vm_area_struct *vma;
- struct task_struct *task;
struct mm_struct *mm;
int rc;
rc = -ENOENT;
- task = get_proc_task(d_inode(dentry));
- if (!task)
- goto out;
-
mm = get_task_mm(task);
- put_task_struct(task);
if (!mm)
goto out;
diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 05c7513e77c7..0f9a1556f2a3 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -171,24 +171,19 @@ static const struct dentry_operations tid_fd_dentry_operations = {
.d_delete = pid_delete_dentry,
};
-static int proc_fd_link(struct dentry *dentry, struct path *path)
+static int proc_fd_link(struct dentry *dentry, struct path *path,
+ struct task_struct *task)
{
- struct task_struct *task;
int ret = -ENOENT;
-
- task = get_proc_task(d_inode(dentry));
- if (task) {
- unsigned int fd = proc_fd(d_inode(dentry));
- struct file *fd_file;
-
- fd_file = fget_task(task, fd);
- if (fd_file) {
- *path = fd_file->f_path;
- path_get(&fd_file->f_path);
- ret = 0;
- fput(fd_file);
- }
- put_task_struct(task);
+ unsigned int fd = proc_fd(d_inode(dentry));
+ struct file *fd_file;
+
+ fd_file = fget_task(task, fd);
+ if (fd_file) {
+ *path = fd_file->f_path;
+ path_get(&fd_file->f_path);
+ ret = 0;
+ fput(fd_file);
}
return ret;
diff --git a/fs/proc/internal.h b/fs/proc/internal.h
index 64dc44832808..d31984c3c797 100644
--- a/fs/proc/internal.h
+++ b/fs/proc/internal.h
@@ -107,7 +107,7 @@ extern struct kmem_cache *proc_dir_entry_cache;
void pde_free(struct proc_dir_entry *pde);
union proc_op {
- int (*proc_get_link)(struct dentry *, struct path *);
+ int (*proc_get_link)(struct dentry *, struct path *, struct task_struct *);
int (*proc_show)(struct seq_file *m,
struct pid_namespace *ns, struct pid *pid,
struct task_struct *task);
--
2.54.0.563.g4f69b47b94-goog