On Wed, Apr 19, 2023 at 11:07:04AM -0400, Waiman Long wrote: > On 4/18/23 23:46, Matthew Wilcox wrote: > > On Tue, Apr 18, 2023 at 09:16:37PM -0400, Waiman Long wrote: > > > 1) App runs creating lots of threads. > > > 2) It mmap's 256K pages of anonymous memory. > > > 3) It writes executable code to that memory. > > > 4) It calls mprotect() with PROT_EXEC on that memory so > > > it can subsequently execute the code. > > > > > > The above mprotect() will fail if the mmap'd region's VMA gets merged with > > > the VMA for one of the thread stacks. That's because the default RHEL > > > SELinux policy is to not allow executable stacks. > > By the way, this is a daft policy. The policy you really want is > > EXEC|WRITE is not allowed. A non-writable stack is useless, so it's > > actually a superset of your current policy. Forbidding _simultaneous_ > > write and executable is just good programming. This way, you don't need > > to care about the underlying VMA's current permissions, you just need > > to do: > > > > if ((prot & (PROT_EXEC|PROT_WRITE)) == (PROT_EXEC|PROT_WRITE)) > > return -EACCESS; > > I am not totally sure if the application changes the VMA to read-only first. > Even if it does that, it highlights another possible issue when an anonymous > VMA is merged with a stack VMA. Either the mprotect() to write-protect the > VMA will fail or the application will segfault if it writes stuff to the > stack. This particular issue is not related to SELinux. It provides another > good idea why we should avoid merging stack VMA to anonymous VMA. mprotect will split the VMA into two VMAs, one that is PROT_READ|PROT_WRITE and one the is PROT_READ|PROT_EXEC.
