On 2023-07-13 22:19:04+1000, Aleksa Sarai wrote:
> Due to an oversight in commit 1b3044e39a89 ("procfs: fix pthread
> cross-thread naming if !PR_DUMPABLE") in switching from REG to NOD,
> chmod operations on /proc/thread-self/comm were no longer blocked as
> they are on almost all other procfs files.
>
> A very similar situation with /proc/self/environ was used to as a root
> exploit a long time ago, but procfs has SB_I_NOEXEC so this is simply a
> correctness issue.
>
> Ref: https://lwn.net/Articles/191954/
> Ref: 6d76fa58b050 ("Don't allow chmod() on the /proc/<pid>/ files")
> Fixes: 1b3044e39a89 ("procfs: fix pthread cross-thread naming if !PR_DUMPABLE")
> Cc: stable@xxxxxxxxxxxxxxx # v4.7+
> Signed-off-by: Aleksa Sarai <cyphar@xxxxxxxxxx>
> ---
> fs/proc/base.c | 3 ++-
> tools/testing/selftests/nolibc/nolibc-test.c | 4 ++++
> 2 files changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index 05452c3b9872..7394229816f3 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -3583,7 +3583,8 @@ static int proc_tid_comm_permission(struct mnt_idmap *idmap,
> }
>
> static const struct inode_operations proc_tid_comm_inode_operations = {
> - .permission = proc_tid_comm_permission,
> + .setattr = proc_setattr,
> + .permission = proc_tid_comm_permission,
> };
Given that this seems to be a recurring theme a more systematic
aproach would help.
Something like the following (untested) patch:
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 05452c3b9872..b90f2e9cda66 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2649,6 +2649,7 @@ static struct dentry *proc_pident_instantiate(struct dentry *dentry,
set_nlink(inode, 2); /* Use getattr to fix if necessary */
if (p->iop)
inode->i_op = p->iop;
+ WARN_ON(!inode->i_op->setattr);
if (p->fop)
inode->i_fop = p->fop;
ei->op = p->op;
> /*
> diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
> index 486334981e60..08f0969208eb 100644
> --- a/tools/testing/selftests/nolibc/nolibc-test.c
> +++ b/tools/testing/selftests/nolibc/nolibc-test.c
> @@ -580,6 +580,10 @@ int run_syscall(int min, int max)
> CASE_TEST(chmod_net); EXPECT_SYSZR(proc, chmod("/proc/self/net", 0555)); break;
> CASE_TEST(chmod_self); EXPECT_SYSER(proc, chmod("/proc/self", 0555), -1, EPERM); break;
> CASE_TEST(chown_self); EXPECT_SYSER(proc, chown("/proc/self", 0, 0), -1, EPERM); break;
> + CASE_TEST(chmod_self_comm); EXPECT_SYSER(proc, chmod("/proc/self/comm", 0777), -1, EPERM); break;
> + CASE_TEST(chmod_tid_comm); EXPECT_SYSER(proc, chmod("/proc/thread-self/comm", 0777), -1, EPERM); break;
> + CASE_TEST(chmod_self_environ);EXPECT_SYSER(proc, chmod("/proc/self/environ", 0777), -1, EPERM); break;
> + CASE_TEST(chmod_tid_environ); EXPECT_SYSER(proc, chmod("/proc/thread-self/environ", 0777), -1, EPERM); break;
I'm not a big fan of this, it abuses the nolibc testsuite to test core
kernel functionality.
If this needs to be tested explicitly there is hopefully a better place.
Those existing tests focus on testing functionality provided by nolibc.
The test chmod_net just got removed because it suffered from the same
bug as /proc/thread-self/comm.
> CASE_TEST(chroot_root); EXPECT_SYSZR(euid0, chroot("/")); break;
> CASE_TEST(chroot_blah); EXPECT_SYSER(1, chroot("/proc/self/blah"), -1, ENOENT); break;
> CASE_TEST(chroot_exe); EXPECT_SYSER(proc, chroot("/proc/self/exe"), -1, ENOTDIR); break;
> --
> 2.41.0
>
