A BPF program can keep dereferencing a trusted pointer that it loaded from a referenced object after it has released that reference. With a file from bpf_get_task_exe_file(), f->f_inode stays usable after bpf_put_file(), and loads through it read freed or reused inode memory. A pointer derived from a referenced object is only valid while the reference is held. release_reference() enforces that by invalidating every register whose id or parent_id matches the released id. However, check_ptr_to_btf_access() builds the loaded register with mark_btf_ld_reg(), which is not given the source register and zeroes both fields. The verifier state for the f_inode case: file = bpf_get_task_exe_file(task) file: id=N, referenced inode = file->f_inode inode: trusted, id=0, parent_id=0 bpf_put_file(file) release_reference(N) skips inode inode->i_ino accepted, unprobed load Reproduced by reading inode->i_ino after bpf_put_file() once the task owning the file had exited and new fds were created. Fix this by recording the source's reference id in the parent_id of a trusted PTR_TO_BTF_ID load. A program that dereferences such a pointer after the release is now rejected with "invalid mem access 'scalar'". Assisted-by: kres Fixes: 57539b1c0ac2 ("bpf: Enable annotating trusted nested pointers") Signed-off-by: Chris Mason --- kernel/bpf/verifier.c | 16 ++++++++++++++++ 1 file changed, 16 insertions(+) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 6c6b8d8520cd..8b48bf796823 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -6219,9 +6219,25 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env, } if (atype == BPF_READ && value_regno >= 0) { + u32 link_id = 0; + + /* + * A trusted pointer walked from a referenced object is only + * valid while that reference is held. Remember the reference + * id of the source, or the one it inherited, so that + * release_reference() invalidates the loaded register too. + * Read it now, reg may alias regs[value_regno]. + */ + if (ret == PTR_TO_BTF_ID && (flag & PTR_TRUSTED)) + link_id = reg_is_referenced(env, reg) ? reg->id : reg->parent_id; + ret = mark_btf_ld_reg(env, regs, value_regno, ret, reg->btf, btf_id, flag); if (ret < 0) return ret; + + /* mark_btf_ld_reg() cleared parent_id, set it afterwards. */ + if (link_id) + regs[value_regno].parent_id = link_id; } return 0; -- 2.54.0