On Thu, Jan 02, 2025 at 03:32:50PM -0800, Isaac J. Manjarres wrote: > Android currently uses the ashmem driver [1] for creating shared memory > regions between processes. Ashmem buffers can initially be mapped with > PROT_READ, PROT_WRITE, and PROT_EXEC. Processes can then use the > ASHMEM_SET_PROT_MASK ioctl command to restrict--never add--the > permissions that the buffer can be mapped with. > > Processes can remove the ability to map ashmem buffers as executable to > ensure that those buffers cannot be exploited to run unintended code. > > For instance, suppose process A allocates a memfd that is meant to be > read and written by itself and another process, call it B. > > Process A shares the buffer with process B, but process B injects code > into the buffer, and compromises process A, such that it makes A map > the buffer with PROT_EXEC. This provides an opportunity for process A > to run the code that process B injected into the buffer. > > If process A had the ability to seal the buffer against future > executable mappings before sharing the buffer with process B, this > attack would not be possible. > > Android is currently trying to replace ashmem with memfd. However, memfd > does not have a provision to permanently remove the ability to map a > buffer as executable, and leaves itself open to the type of attack > described earlier. However, this should be something that can be > achieved via a new file seal. > > There are known usecases (e.g. CursorWindow [2]) where a process > maps a buffer with read/write permissions before restricting the buffer > to being mapped as read-only for future mappings. > > The resulting VMA from the writable mapping has VM_MAYEXEC set, meaning > that mprotect() can change the mapping to be executable. Therefore, > implementing the seal similar to F_SEAL_WRITE would not be appropriate, > since it would not work with the CursorWindow usecase. This is because > the CursorWindow process restricts the mapping permissions to read-only > after the writable mapping is created. So, adding a file seal for > executable mappings that operates like F_SEAL_WRITE would fail. > > Therefore, add support for F_SEAL_FUTURE_EXEC, which is handled > similarly to F_SEAL_FUTURE_WRITE. This ensures that CursorWindow can > continue to create a writable mapping initially, and then restrict the > permissions on the buffer to be mappable as read-only by using both > F_SEAL_FUTURE_WRITE and F_SEAL_FUTURE_EXEC. After the seal is > applied, any calls to mmap() with PROT_EXEC will fail. > > [1] https://cs.android.com/android/kernel/superproject/+/common-android-mainline:common/drivers/staging/android/ashmem.c > [2] https://developer.android.com/reference/android/database/CursorWindow > > Signed-off-by: Isaac J. Manjarres <isaacmanjarres@xxxxxxxxxx> > --- > include/uapi/linux/fcntl.h | 1 + > mm/memfd.c | 39 +++++++++++++++++++++++++++++++++++++- > 2 files changed, 39 insertions(+), 1 deletion(-) > > diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h > index 6e6907e63bfc..ef066e524777 100644 > --- a/include/uapi/linux/fcntl.h > +++ b/include/uapi/linux/fcntl.h > @@ -49,6 +49,7 @@ > #define F_SEAL_WRITE 0x0008 /* prevent writes */ > #define F_SEAL_FUTURE_WRITE 0x0010 /* prevent future writes while mapped */ > #define F_SEAL_EXEC 0x0020 /* prevent chmod modifying exec bits */ Hmm ok I just noticed this... F_SEAL_EXEC is weird then. It doesn't prevent execution in the same way F_SEAL_WRITE does, nor does it seem to check or care about VM_MAYEXEC... It just 'prevents chmod from modifying exec bits'. I mean lord above haha. And of course the code for it is in shmem_setattr()... I have not enough faces to palm or palms to face. So yes I suppose for any sane exec semantics you'll need something new... > +#define F_SEAL_FUTURE_EXEC 0x0040 /* prevent future executable mappings */ > /* (1U << 31) is reserved for signed error codes */ > > /* > diff --git a/mm/memfd.c b/mm/memfd.c > index 5f5a23c9051d..cfd62454df5e 100644 > --- a/mm/memfd.c > +++ b/mm/memfd.c > @@ -184,6 +184,7 @@ static unsigned int *memfd_file_seals_ptr(struct file *file) > } > > #define F_ALL_SEALS (F_SEAL_SEAL | \ > + F_SEAL_FUTURE_EXEC |\ > F_SEAL_EXEC | \ > F_SEAL_SHRINK | \ > F_SEAL_GROW | \ > @@ -357,14 +358,50 @@ static int check_write_seal(unsigned long *vm_flags_ptr) > return 0; > } > > +static inline bool is_exec_sealed(unsigned int seals) This should say 'future', otherwise this is very confusing vs. F_SEAL_EXEC. Also no need for inline outside of a header. > +{ > + return seals & F_SEAL_FUTURE_EXEC; > +} > + > +static int check_exec_seal(unsigned long *vm_flags_ptr) > +{ > + unsigned long vm_flags = *vm_flags_ptr; > + unsigned long mask = vm_flags & (VM_SHARED | VM_EXEC); > + > + /* Executability is not a concern for private mappings. */ > + if (!(mask & VM_SHARED)) > + return 0; > + > + /* > + * New PROT_EXEC and MAP_SHARED mmaps are not allowed when exec seal > + * is active. > + */ > + if (mask & VM_EXEC) > + return -EPERM; > + > + /* > + * Prevent mprotect() from making an exec-sealed mapping executable in > + * the future. > + */ > + *vm_flags_ptr &= ~VM_MAYEXEC; > + > + return 0; > +} > + > int memfd_check_seals_mmap(struct file *file, unsigned long *vm_flags_ptr) > { > int err = 0; > unsigned int *seals_ptr = memfd_file_seals_ptr(file); > unsigned int seals = seals_ptr ? *seals_ptr : 0; > > - if (is_write_sealed(seals)) > + if (is_write_sealed(seals)) { > err = check_write_seal(vm_flags_ptr); > + if (err) > + return err; > + } > + > + if (is_exec_sealed(seals)) > + err = check_exec_seal(vm_flags_ptr); > > return err; > } OK this is actually quite neat now we have everything set up in do_mmap(). I think we probably want some comments to very clearly point out that F_SEAL_EXEC is a bit crazy and weird and meaningless and this is actually vaguely sane... > -- > 2.47.1.613.gc27f4b7a9f-goog >
