Description
In the Linux kernel, the following vulnerability has been resolved: unshare: fix unshare_fs() handling There's an unpleasant corner case in unshare(2), when we have a CLONE_NEWNS in flags and current->fs hadn't been shared at all; in that case copy_mnt_ns() gets passed current->fs instead of a private copy, which causes interesting warts in proof of correctness] > I guess if private means fs->users == 1, the condition could still be true. Unfortunately, it's worse than just a convoluted proof of correctness. Consider the case when we have CLONE_NEWCGROUP in addition to CLONE_NEWNS (and current->fs->users == 1). We pass current->fs to copy_mnt_ns(), all right. Suppose it succeeds and flips current->fs->{pwd,root} to corresponding locations in the new namespace. Now we proceed to copy_cgroup_ns(), which fails (e.g. with -ENOMEM). We call put_mnt_ns() on the namespace created by copy_mnt_ns(), it's destroyed and its mount tree is dissolved, but... current->fs->root and current->fs->pwd are both left pointing to now detached mounts. They are pinning those, so it's not a UAF, but it leaves the calling process with unshare(2) failing with -ENOMEM _and_ leaving it with pwd and root on detached isolated mounts. The last part is clearly a bug. There is other fun related to that mess (races with pivot_root(), including the one between pivot_root() and fork(), of all things), but this one is easy to isolate and fix - treat CLONE_NEWNS as "allocate a new fs_struct even if it hadn't been shared in the first place". Sure, we could go for something like "if both CLONE_NEWNS *and* one of the things that might end up failing after copy_mnt_ns() call in create_new_namespaces() are set, force allocation of new fs_struct", but let's keep it simple - the cost of copy_fs_struct() is trivial. Another benefit is that copy_mnt_ns() with CLONE_NEWNS *always* gets a freshly allocated fs_struct, yet to be attached to anything. That seriously simplifies the analysis... FWIW, that bug had been there since the introduction of unshare(2) ;-/
Product status
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (git) before 845bf3c6963a52096d0d3866e4a92db77a0c03d8
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (git) before d3ffc8f13034af895531a02c30b1fe3a34b46432
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (git) before d0d99f60538ddb4a62ccaac2168d8f448965f083
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (git) before d7963d6997fea86a6def242ac36198b86655f912
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (git) before aa9ebc084505fb26dd90f4d7a249045aad152043
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (git) before af8f4be3b68ac8caa41c8e5ead0eeaf5e85e42d0
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (git) before 42e21e74061b0ebbd859839f81acf10efad02a27
1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (git) before 6c4b2243cb6c0755159bd567130d5e12e7b10d9f
5.10.253 (semver)
5.15.203 (semver)
6.1.167 (semver)
6.6.130 (semver)
6.12.78 (semver)
6.18.19 (semver)
6.19.9 (semver)
7.0 (original_commit_for_fix)
References
git.kernel.org/...c/845bf3c6963a52096d0d3866e4a92db77a0c03d8
git.kernel.org/...c/d3ffc8f13034af895531a02c30b1fe3a34b46432
git.kernel.org/...c/d0d99f60538ddb4a62ccaac2168d8f448965f083
git.kernel.org/...c/d7963d6997fea86a6def242ac36198b86655f912
git.kernel.org/...c/aa9ebc084505fb26dd90f4d7a249045aad152043
git.kernel.org/...c/af8f4be3b68ac8caa41c8e5ead0eeaf5e85e42d0
git.kernel.org/...c/42e21e74061b0ebbd859839f81acf10efad02a27
git.kernel.org/...c/6c4b2243cb6c0755159bd567130d5e12e7b10d9f