Messages in this thread | | | Subject | Re: [PATCH v1 3/7] kernel/fork: always deny write access to current MM exe_file | From | David Hildenbrand <> | Date | Thu, 12 Aug 2021 12:13:44 +0200 |
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On 12.08.21 12:05, Christian Brauner wrote: > [+Cc Andrei] > > On Thu, Aug 12, 2021 at 10:43:44AM +0200, David Hildenbrand wrote: >> We want to remove VM_DENYWRITE only currently only used when mapping the >> executable during exec. During exec, we already deny_write_access() the >> executable, however, after exec completes the VMAs mapped >> with VM_DENYWRITE effectively keeps write access denied via >> deny_write_access(). >> >> Let's deny write access when setting the MM exe_file. With this change, we >> can remove VM_DENYWRITE for mapping executables. >> >> This represents a minor user space visible change: >> sys_prctl(PR_SET_MM_EXE_FILE) can now fail if the file is already >> opened writable. Also, after sys_prctl(PR_SET_MM_EXE_FILE), the file > > Just for completeness, this also affects PR_SET_MM_MAP when exe_fd is > set.
Correct.
> >> cannot be opened writable. Note that we can already fail with -EACCES if >> the file doesn't have execute permissions. >> >> Signed-off-by: David Hildenbrand <david@redhat.com> >> --- > > The biggest user I know and that I'm involved in is CRIU which heavily > uses PR_SET_MM_MAP (with a fallback to PR_SET_MM_EXE_FILE on older > kernels) during restore. Afair, criu opens the exe fd as an O_PATH > during dump and thus will use the same flag during restore when > opening it. So that should be fine.
Yes.
> > However, if I understand the consequences of this change correctly, a > problem could be restoring workloads that hold a writable fd open to > their exe file at dump time which would mean that during restore that fd > would be reopened writable causing CRIU to fail when setting the exe > file for the task to be restored.
If it's their exe file, then the existing VM_DENYWRITE handling would have forbidden these workloads to open the fd of their exe file writable, right? At least before doing any PR_SET_MM_MAP/PR_SET_MM_EXE_FILE. But that should rule out quite a lot of cases we might be worried about, right?
> > Which honestly, no idea how many such workloads exist. (I know at least > of runC and LXC need to sometimes reopen to rexec themselves (weird bug > to protect against attacking the exe file) and thus re-open > /proc/self/exe but read-only.) > >> kernel/fork.c | 39 ++++++++++++++++++++++++++++++++++----- >> 1 file changed, 34 insertions(+), 5 deletions(-) >> >> diff --git a/kernel/fork.c b/kernel/fork.c >> index 6bd2e52bcdfb..5d904878f19b 100644 >> --- a/kernel/fork.c >> +++ b/kernel/fork.c >> @@ -476,6 +476,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, >> { >> struct vm_area_struct *mpnt, *tmp, *prev, **pprev; >> struct rb_node **rb_link, *rb_parent; >> + struct file *exe_file; >> int retval; >> unsigned long charge; >> LIST_HEAD(uf); >> @@ -493,7 +494,10 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, >> mmap_write_lock_nested(mm, SINGLE_DEPTH_NESTING); >> >> /* No ordering required: file already has been exposed. */ >> - RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm)); >> + exe_file = get_mm_exe_file(oldmm); >> + RCU_INIT_POINTER(mm->exe_file, exe_file); >> + if (exe_file) >> + deny_write_access(exe_file); >> >> mm->total_vm = oldmm->total_vm; >> mm->data_vm = oldmm->data_vm; >> @@ -638,8 +642,13 @@ static inline void mm_free_pgd(struct mm_struct *mm) >> #else >> static int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm) >> { >> + struct file *exe_file; >> + >> mmap_write_lock(oldmm); >> - RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm)); >> + exe_file = get_mm_exe_file(oldmm); >> + RCU_INIT_POINTER(mm->exe_file, exe_file); >> + if (exe_file) >> + deny_write_access(exe_file); >> mmap_write_unlock(oldmm); >> return 0; >> } >> @@ -1163,11 +1172,19 @@ void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file) >> */ >> old_exe_file = rcu_dereference_raw(mm->exe_file); >> >> - if (new_exe_file) >> + if (new_exe_file) { >> get_file(new_exe_file); >> + /* >> + * exec code is required to deny_write_access() successfully, >> + * so this cannot fail >> + */ >> + deny_write_access(new_exe_file); >> + } >> rcu_assign_pointer(mm->exe_file, new_exe_file); >> - if (old_exe_file) >> + if (old_exe_file) { >> + allow_write_access(old_exe_file); >> fput(old_exe_file); >> + } >> } >> >> int atomic_set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file) >> @@ -1194,10 +1211,22 @@ int atomic_set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file) >> } >> >> /* set the new file, lockless */ >> + ret = deny_write_access(new_exe_file); >> + if (ret) >> + return -EACCES; >> get_file(new_exe_file); >> + >> old_exe_file = xchg(&mm->exe_file, new_exe_file); >> - if (old_exe_file) >> + if (old_exe_file) { >> + /* >> + * Don't race with dup_mmap() getting the file and disallowing >> + * write access while someone might open the file writable. >> + */ >> + mmap_read_lock(mm); >> + allow_write_access(old_exe_file); >> fput(old_exe_file); >> + mmap_read_unlock(mm); >> + } >> return 0; >> } >> >> -- >> 2.31.1 >> >
-- Thanks,
David / dhildenb
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