This effectively reverts commit 8ee74a91ac ("proc: try to remove use
of FOLL_FORCE entirely")
It turns out that people do depend on FOLL_FORCE for the /proc/<pid>/mem
case, and we're talking not just debuggers. Talking to the affected people, the use-cases are:
Keno Fischer:
"We used these semantics as a hardening mechanism in the julia JIT. By
opening /proc/self/mem and using these semantics, we could avoid
needing RWX pages, or a dual mapping approach. We do have fallbacks to
these other methods (though getting EIO here actually causes an assert
in released versions - we'll updated that to make sure to take the
fall back in that case).
Nevertheless the /proc/self/mem approach was our favored approach
because it a) Required an attacker to be able to execute syscalls
which is a taller order than getting memory write and b) didn't double
the virtual address space requirements (as a dual mapping approach
would).
I think in general this feature is very useful for anybody who needs
to precisely control the execution of some other process. Various
debuggers (gdb/lldb/rr) certainly fall into that category, but there's
another class of such processes (wine, various emulators) which may
want to do that kind of thing.
Now, I suspect most of these will have the other process under ptrace
control, so maybe allowing (same_mm || ptraced) would be ok, but at
least for the sandbox/remote-jit use case, it would be perfectly
reasonable to not have the jit server be a ptracer"
Robert O'Callahan:
"We write to readonly code and data mappings via /proc/.../mem in lots
of different situations, particularly when we're adjusting program
state during replay to match the recorded execution.
Like Julia, we can add workarounds, but they could be expensive."
so not only do people use FOLL_FORCE for both reads and writes, but they
use it for both the local mm and remote mm.
With these comments in mind, we likely also cannot add the "are we
actively ptracing" check either, so this keeps the new code organization
and does not do a real revert that would add back the original comment
about "Maybe we should limit FOLL_FORCE to actual ptrace users?"
Reported-by: Keno Fischer <keno@juliacomputing.com>
Reported-by: Robert O'Callahan <robert@ocallahan.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Eric Biederman <ebiederm@xmission.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
f511c0b17b