[PATCH RFC 0/4] mm, arm64: In-kernel support for memory-deny-write-execute (MDWE)

Catalin Marinas
2022-04-13T13:49:57Z

Hi,

The background to this is that systemd has a configuration option called
MemoryDenyWriteExecute [1], implemented as a SECCOMP BPF filter. Its aim
is to prevent a user task from inadvertently creating an executable
mapping that is (or was) writeable. Since such BPF filter is stateless,
it cannot detect mappings that were previously writeable but
subsequently changed to read-only. Therefore the filter simply rejects
any mprotect(PROT_EXEC). The side-effect is that on arm64 with BTI
support (Branch Target Identification), the dynamic loader cannot change
an ELF section from PROT_EXEC to PROT_EXEC|PROT_BTI using mprotect().
For libraries, it can resort to unmapping and re-mapping but for the
main executable it does not have a file descriptor. The original bug
report in the Red Hat bugzilla - [2] - and subsequent glibc workaround
for libraries - [3].

Add in-kernel support for such feature as a DENY_WRITE_EXEC personality
flag, inherited on fork() and execve(). The kernel tracks a previously
writeable mapping via a new VM_WAS_WRITE flag (64-bit only
architectures). I went for a personality flag by analogy with the
READ_IMPLIES_EXEC one. However, I'm happy to change it to a prctl() if
we don't want more personality flags. A minor downside with the
personality flag is that there is no way for the user to query which
flags are supported, so in patch 3 I added an AT_FLAGS bit to advertise
this.

Posting this as an RFC to start a discussion and cc'ing some of the
systemd guys and those involved in the earlier thread around the glibc
workaround for dynamic libraries [4]. Before thinking of upstreaming
this we'd need the systemd folk to buy into replacing the MDWE SECCOMP
BPF filter with the in-kernel one.

Thanks,

Catalin

[1] https://www.freedesktop.org/software/systemd/man/systemd.exec.html#MemoryDenyWriteExecute=
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1888842
[3] https://sourceware.org/bugzilla/show_bug.cgi?id=26831
[3] https://lore.kernel.org/r/cover.1604393169.git.szabolcs.nagy@arm.com

Catalin Marinas (4):
mm: Track previously writeable vma permission
mm, personality: Implement memory-deny-write-execute as a personality
flag
fs/binfmt_elf: Tell user-space about the DENY_WRITE_EXEC personality
flag
arm64: Select ARCH_ENABLE_DENY_WRITE_EXEC

arch/arm64/Kconfig | 1 +
fs/binfmt_elf.c | 2 ++
include/linux/mm.h | 6 ++++++
include/linux/mman.h | 18 +++++++++++++++++-
include/uapi/linux/binfmts.h | 4 ++++
include/uapi/linux/personality.h | 1 +
mm/Kconfig | 4 ++++
mm/mmap.c | 3 +++
mm/mprotect.c | 5 +++++
9 files changed, 43 insertions(+), 1 deletion(-)