From: "Tyler Hicks (Microsoft)" code@tyhicks.com

The following commits loosened the permissions of /proc/<PID>/fdinfo/ directory, as well as the files within it, from 0500 to 0555 while also introducing a PTRACE_MODE_READ check between the current task and <PID>'s task:

- commit 7bc3fa0172a4 ("procfs: allow reading fdinfo with PTRACE_MODE_READ") - commit 1927e498aee1 ("procfs: prevent unprivileged processes accessing fdinfo dir")

Before those changes, inode based system calls like inotify_add_watch(2) would fail when the current task didn't have sufficient read permissions:

[...] lstat("/proc/1/task/1/fdinfo", {st_mode=S_IFDIR|0500, st_size=0, ...}) = 0 inotify_add_watch(64, "/proc/1/task/1/fdinfo", IN_MODIFY|IN_ATTRIB|IN_MOVED_FROM|IN_MOVED_TO|IN_CREATE|IN_DELETE| IN_ONLYDIR|IN_DONT_FOLLOW|IN_EXCL_UNLINK) = -1 EACCES (Permission denied) [...]

This matches the documented behavior in the inotify_add_watch(2) man page:

ERRORS EACCES Read access to the given file is not permitted.

After those changes, inotify_add_watch(2) started succeeding despite the current task not having PTRACE_MODE_READ privileges on the target task:

[...] lstat("/proc/1/task/1/fdinfo", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0 inotify_add_watch(64, "/proc/1/task/1/fdinfo", IN_MODIFY|IN_ATTRIB|IN_MOVED_FROM|IN_MOVED_TO|IN_CREATE|IN_DELETE| IN_ONLYDIR|IN_DONT_FOLLOW|IN_EXCL_UNLINK) = 1757 openat(AT_FDCWD, "/proc/1/task/1/fdinfo", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = -1 EACCES (Permission denied) [...]

This change in behavior broke .NET prior to v7. See the github link below for the v7 commit that inadvertently/quietly (?) fixed .NET after the kernel changes mentioned above.

Return to the old behavior by moving the PTRACE_MODE_READ check out of the file .open operation and into the inode .permission operation:

[...] lstat("/proc/1/task/1/fdinfo", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0 inotify_add_watch(64, "/proc/1/task/1/fdinfo", IN_MODIFY|IN_ATTRIB|IN_MOVED_FROM|IN_MOVED_TO|IN_CREATE|IN_DELETE| IN_ONLYDIR|IN_DONT_FOLLOW|IN_EXCL_UNLINK) = -1 EACCES (Permission denied) [...]

Reported-by: Kevin Parsons (Microsoft) parsonskev@gmail.com Link: https://github.com/dotnet/runtime/commit/89e5469ac591b82d38510fe7de98346cce7... Link: https://stackoverflow.com/questions/75379065/start-self-contained-net6-build... Fixes: 7bc3fa0172a4 ("procfs: allow reading fdinfo with PTRACE_MODE_READ") Cc: stable@vger.kernel.org Cc: Christian Brauner brauner@kernel.org Cc: "Christian König" christian.koenig@amd.com Cc: Jann Horn jannh@google.com Cc: Kalesh Singh kaleshsingh@google.com Cc: Hardik Garg hargar@linux.microsoft.com Cc: Allen Pais apais@linux.microsoft.com Signed-off-by: Tyler Hicks (Microsoft) code@tyhicks.com --- fs/proc/fd.c | 42 ++++++++++++++++++++---------------------- 1 file changed, 20 insertions(+), 22 deletions(-)

diff --git a/fs/proc/fd.c b/fs/proc/fd.c index 6e72e5ad42bc..f4b1c8b42a51 100644 --- a/fs/proc/fd.c +++ b/fs/proc/fd.c @@ -74,7 +74,18 @@ static int seq_show(struct seq_file *m, void *v) return 0; }

-static int proc_fdinfo_access_allowed(struct inode *inode) +static int seq_fdinfo_open(struct inode *inode, struct file *file) +{ + return single_open(file, seq_show, inode); +} + +/** + * Shared /proc/pid/fdinfo and /proc/pid/fdinfo/fd permission helper to ensure + * that the current task has PTRACE_MODE_READ in addition to the normal + * POSIX-like checks. + */ +static int proc_fdinfo_permission(struct mnt_idmap *idmap, struct inode *inode, + int mask) { bool allowed = false; struct task_struct *task = get_proc_task(inode); @@ -88,18 +99,13 @@ static int proc_fdinfo_access_allowed(struct inode *inode) if (!allowed) return -EACCES;

- return 0; + return generic_permission(idmap, inode, mask); }

-static int seq_fdinfo_open(struct inode *inode, struct file *file) -{ - int ret = proc_fdinfo_access_allowed(inode); - - if (ret) - return ret; - - return single_open(file, seq_show, inode); -} +static const struct inode_operations proc_fdinfo_file_inode_operations = { + .permission = proc_fdinfo_permission, + .setattr = proc_setattr, +};

static const struct file_operations proc_fdinfo_file_operations = { .open = seq_fdinfo_open, @@ -388,6 +394,8 @@ static struct dentry *proc_fdinfo_instantiate(struct dentry *dentry, ei = PROC_I(inode); ei->fd = data->fd;

+ inode->i_op = &proc_fdinfo_file_inode_operations; + inode->i_fop = &proc_fdinfo_file_operations; tid_fd_update_inode(task, inode, 0);

@@ -407,23 +415,13 @@ static int proc_readfdinfo(struct file *file, struct dir_context *ctx) proc_fdinfo_instantiate); }

-static int proc_open_fdinfo(struct inode *inode, struct file *file) -{ - int ret = proc_fdinfo_access_allowed(inode); - - if (ret) - return ret; - - return 0; -} - const struct inode_operations proc_fdinfo_inode_operations = { .lookup = proc_lookupfdinfo, + .permission = proc_fdinfo_permission, .setattr = proc_setattr, };

const struct file_operations proc_fdinfo_operations = { - .open = proc_open_fdinfo, .read = generic_read_dir, .iterate_shared = proc_readfdinfo, .llseek = generic_file_llseek,