On Mon 07-12-15 16:40:14, Kees Cook wrote: > On Mon, Dec 7, 2015 at 2:42 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > On Thu, Dec 3, 2015 at 5:45 PM, yalin wang <yalin.wang2010@xxxxxxxxx> wrote: > >> > >>> On Dec 2, 2015, at 16:03, Kees Cook <keescook@xxxxxxxxxxxx> wrote: > >>> > >>> Normally, when a user can modify a file that has setuid or setgid bits, > >>> those bits are cleared when they are not the file owner or a member > >>> of the group. This is enforced when using write and truncate but not > >>> when writing to a shared mmap on the file. This could allow the file > >>> writer to gain privileges by changing a binary without losing the > >>> setuid/setgid/caps bits. > >>> > >>> Changing the bits requires holding inode->i_mutex, so it cannot be done > >>> during the page fault (due to mmap_sem being held during the fault). > >>> Instead, clear the bits if PROT_WRITE is being used at mmap time. > >>> > >>> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> > >>> Cc: stable@xxxxxxxxxxxxxxx > >>> — > >> > >> is this means mprotect() sys call also need add this check? > >> mprotect() can change to PROT_WRITE, then it can write to a > >> read only map again , also a secure hole here . > > > > Yes, good point. This needs to be added. I will send a new patch. Thanks! > > This continues to look worse and worse. > > So... to check this at mprotect time, I have to know it's MAP_SHARED, > but that's in the vma_flags, which I can only see after holding > mmap_sem. > > The best I can think of now is to strip the bits at munmap time, since > you can't execute an mmapped file until it closes. > > Jan, thoughts on this? Umm, so we actually refuse to execute a file while someone has it open for writing (deny_write_access() in do_open_execat()). So dropping the suid / sgid bits when closing file for writing could be plausible. Grabbing i_mutex from __fput() context is safe (it gets called from task_work context when returning to userspace). That way we could actually remove the checks done for each write. To avoid unexpected removal of suid/sgid bits when someone just opens & closes the file, we could mark the file as needing suid/sgid treatment by a flag in inode->i_flags when file gets written to or mmaped and then check for this in __fput(). I've added Al Viro to CC just in case he is aware of some issues with this... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>