On 06/19/2012 10:24 AM, Serge Hallyn wrote: > Quoting Wanlong Gao (gaowanlong@xxxxxxxxxxxxxx): >> On 03/29/2012 05:55 PM, tip-bot for Kees Cook wrote: >>> Commit-ID: bdbb776f882f5ad431aa1e694c69c1c3d6a4a5b8 >>> Gitweb: http://git.kernel.org/tip/bdbb776f882f5ad431aa1e694c69c1c3d6a4a5b8 >>> Author: Kees Cook <keescook@xxxxxxxxxxxx> >>> AuthorDate: Mon, 19 Mar 2012 16:12:53 -0700 >>> Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx> >>> CommitDate: Thu, 29 Mar 2012 11:37:17 +0200 >>> >>> futex: Do not leak robust list to unprivileged process >>> >>> It was possible to extract the robust list head address from a setuid >>> process if it had used set_robust_list(), allowing an ASLR info leak. This >>> changes the permission checks to be the same as those used for similar >>> info that comes out of /proc. >>> >>> Running a setuid program that uses robust futexes would have had: >>> cred->euid != pcred->euid >>> cred->euid == pcred->uid >>> so the old permissions check would allow it. I'm not aware of any setuid >>> programs that use robust futexes, so this is just a preventative measure. >>> >> >> I'm not sure this change prevents the unprivileged process. >> Please refer to LTP test, recently I saw that this change broke >> the following test. >> >> https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/syscalls/get_robust_list/get_robust_list01.c#L155 >> if (seteuid(1) == -1) >> tst_brkm(TBROK|TERRNO, cleanup, "seteuid(1) failed"); >> >> TEST(retval = syscall(__NR_get_robust_list, 1, >> (struct robust_list_head *)&head, >> &len_ptr)); >> >> We set the euid to an unprivileged user, and expect to FAIL with EPERM, >> without this patch, it FAIL as we expected, but with it, this call succeed. > > This relates to a question I asked - I believe in this thread, maybe in > another thread - about ptrace_may_access. That code goes back further than > our git history, and for so long has used current->uid and ->gid, not > euid and gid, for permission checks. I asked if that's what we really > want, but at the same am not sure we want to change something that's > been like that for so long. > > But that's why it succeeded - you changed your euid, not your uid. Yeah, I known what I'm doing. I just wonder which is the right thing. Should we check euid or uid ? You mean that checking uid instead of checking euid for a long time, right? Thanks, Wanlong Gao > >> Seems that we leaked the check of (cred->euid == pcred->euid && cred->euid == pcred->uid), >> I'm not sure which one is right, can you please give an explanation? >> >> >> Thanks in advance, >> Wanlong Gao >> >>> (This patch is based on changes from grsecurity.) >>> >>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> >>> Cc: Darren Hart <dvhart@xxxxxxxxxxxxxxx> >>> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> >>> Cc: Jiri Kosina <jkosina@xxxxxxx> >>> Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx> >>> Cc: David Howells <dhowells@xxxxxxxxxx> >>> Cc: Serge E. Hallyn <serge.hallyn@xxxxxxxxxxxxx> >>> Cc: kernel-hardening@xxxxxxxxxxxxxxxxxx >>> Cc: spender@xxxxxxxxxxxxxx >>> Link: http://lkml.kernel.org/r/20120319231253.GA20893@xxxxxxxxxxxxxxx >>> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> >>> --- >>> kernel/futex.c | 36 +++++++++++++----------------------- >>> kernel/futex_compat.c | 36 +++++++++++++----------------------- >>> 2 files changed, 26 insertions(+), 46 deletions(-) >>> >>> diff --git a/kernel/futex.c b/kernel/futex.c >>> index 72efa1e..d701be5 100644 >>> --- a/kernel/futex.c >>> +++ b/kernel/futex.c >>> @@ -59,6 +59,7 @@ >>> #include <linux/magic.h> >>> #include <linux/pid.h> >>> #include <linux/nsproxy.h> >>> +#include <linux/ptrace.h> >>> >>> #include <asm/futex.h> >>> >>> @@ -2443,40 +2444,29 @@ SYSCALL_DEFINE3(get_robust_list, int, pid, >>> { >>> struct robust_list_head __user *head; >>> unsigned long ret; >>> - const struct cred *cred = current_cred(), *pcred; >>> + struct task_struct *p; >>> >>> if (!futex_cmpxchg_enabled) >>> return -ENOSYS; >>> >>> + rcu_read_lock(); >>> + >>> + ret = -ESRCH; >>> if (!pid) >>> - head = current->robust_list; >>> + p = current; >>> else { >>> - struct task_struct *p; >>> - >>> - ret = -ESRCH; >>> - rcu_read_lock(); >>> p = find_task_by_vpid(pid); >>> if (!p) >>> goto err_unlock; >>> - ret = -EPERM; >>> - pcred = __task_cred(p); >>> - /* If victim is in different user_ns, then uids are not >>> - comparable, so we must have CAP_SYS_PTRACE */ >>> - if (cred->user->user_ns != pcred->user->user_ns) { >>> - if (!ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE)) >>> - goto err_unlock; >>> - goto ok; >>> - } >>> - /* If victim is in same user_ns, then uids are comparable */ >>> - if (cred->euid != pcred->euid && >>> - cred->euid != pcred->uid && >>> - !ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE)) >>> - goto err_unlock; >>> -ok: >>> - head = p->robust_list; >>> - rcu_read_unlock(); >>> } >>> >>> + ret = -EPERM; >>> + if (!ptrace_may_access(p, PTRACE_MODE_READ)) >>> + goto err_unlock; >>> + >>> + head = p->robust_list; >>> + rcu_read_unlock(); >>> + >>> if (put_user(sizeof(*head), len_ptr)) >>> return -EFAULT; >>> return put_user(head, head_ptr); >>> diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c >>> index 5f9e689..a9642d5 100644 >>> --- a/kernel/futex_compat.c >>> +++ b/kernel/futex_compat.c >>> @@ -10,6 +10,7 @@ >>> #include <linux/compat.h> >>> #include <linux/nsproxy.h> >>> #include <linux/futex.h> >>> +#include <linux/ptrace.h> >>> >>> #include <asm/uaccess.h> >>> >>> @@ -136,40 +137,29 @@ compat_sys_get_robust_list(int pid, compat_uptr_t __user *head_ptr, >>> { >>> struct compat_robust_list_head __user *head; >>> unsigned long ret; >>> - const struct cred *cred = current_cred(), *pcred; >>> + struct task_struct *p; >>> >>> if (!futex_cmpxchg_enabled) >>> return -ENOSYS; >>> >>> + rcu_read_lock(); >>> + >>> + ret = -ESRCH; >>> if (!pid) >>> - head = current->compat_robust_list; >>> + p = current; >>> else { >>> - struct task_struct *p; >>> - >>> - ret = -ESRCH; >>> - rcu_read_lock(); >>> p = find_task_by_vpid(pid); >>> if (!p) >>> goto err_unlock; >>> - ret = -EPERM; >>> - pcred = __task_cred(p); >>> - /* If victim is in different user_ns, then uids are not >>> - comparable, so we must have CAP_SYS_PTRACE */ >>> - if (cred->user->user_ns != pcred->user->user_ns) { >>> - if (!ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE)) >>> - goto err_unlock; >>> - goto ok; >>> - } >>> - /* If victim is in same user_ns, then uids are comparable */ >>> - if (cred->euid != pcred->euid && >>> - cred->euid != pcred->uid && >>> - !ns_capable(pcred->user->user_ns, CAP_SYS_PTRACE)) >>> - goto err_unlock; >>> -ok: >>> - head = p->compat_robust_list; >>> - rcu_read_unlock(); >>> } >>> >>> + ret = -EPERM; >>> + if (!ptrace_may_access(p, PTRACE_MODE_READ)) >>> + goto err_unlock; >>> + >>> + head = p->compat_robust_list; >>> + rcu_read_unlock(); >>> + >>> if (put_user(sizeof(*head), len_ptr)) >>> return -EFAULT; >>> return put_user(ptr_to_compat(head), head_ptr); >>> -- >>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in >>> the body of a message to majordomo@xxxxxxxxxxxxxxx >>> More majordomo info at http://vger.kernel.org/majordomo-info.html >>> Please read the FAQ at http://www.tux.org/lkml/ >>> >> >> > -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html