Messages in this thread | | | Date | Wed, 17 Oct 2018 03:39:58 -0700 | From | Joel Fernandes <> | Subject | Re: [PATCH v2 1/2] mm: Add an F_SEAL_FS_WRITE seal to memfd |
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On Wed, Oct 17, 2018 at 02:51:55AM -0700, Christoph Hellwig wrote: > On Tue, Oct 09, 2018 at 03:20:41PM -0700, Joel Fernandes (Google) wrote: > > One of the main usecases Android has is the ability to create a region > > and mmap it as writeable, then drop its protection for "future" writes > > while keeping the existing already mmap'ed writeable-region active. > > s/drop/add/ ? > > Otherwise this doesn't make much sense to me.
Sure, you are right that "add" is more appropriate. I'll change it to that.
> > This usecase cannot be implemented with the existing F_SEAL_WRITE seal. > > To support the usecase, this patch adds a new F_SEAL_FS_WRITE seal which > > prevents any future mmap and write syscalls from succeeding while > > keeping the existing mmap active. The following program shows the seal > > working in action: > > Where does the FS come from? I'd rather expect this to be implemented > as a 'force' style flag that applies the seal even if the otherwise > required precondition is not met.
The "FS" was meant to convey that the seal is preventing writes at the VFS layer itself, for example vfs_write checks FMODE_WRITE and does not proceed, it instead returns an error if the flag is not set. I could not find a better name for it, I could call it F_SEAL_VFS_WRITE if you prefer?
> > Note: This seal will also prevent growing and shrinking of the memfd. > > This is not something we do in Android so it does not affect us, however > > I have mentioned this behavior of the seal in the manpage. > > This seems odd, as that is otherwise split into the F_SEAL_SHRINK / > F_SEAL_GROW flags.
I could make it such that this seal would not be allowed unless F_SEAL_SHRINK and F_SEAL_GROW are either previously set, or they are passed along with this seal. Would that make more sense to you?
> > static int memfd_add_seals(struct file *file, unsigned int seals) > > { > > @@ -219,6 +220,9 @@ static int memfd_add_seals(struct file *file, unsigned int seals) > > } > > } > > > > + if ((seals & F_SEAL_FS_WRITE) && !(*file_seals & F_SEAL_FS_WRITE)) > > + file->f_mode &= ~(FMODE_WRITE | FMODE_PWRITE); > > + > > This seems to lack any synchronization for f_mode.
The f_mode is set when the struct file is first created and then memfd sets additional flags in memfd_create. Then later we are changing it here at the time of setting the seal. I donot see any possiblity of a race since it is impossible to set the seal before memfd_create returns. Could you provide more details about what kind of synchronization is needed and what is the race condition scenario you were thinking off?
thanks for the review,
- Joel
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