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authorLinus Torvalds <torvalds@linux-foundation.org>2017-05-30 12:38:59 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2017-05-30 12:38:59 -0700
commitf511c0b17b081562dca8ac5061dfa86db4c66cc2 (patch)
treef81ae7331bc9dc2c9f6e448d5159bbb3dcd168d0 /fs/proc
parent3f173bde7e4320211e77a83f936fb754e7591006 (diff)
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"Yes, people use FOLL_FORCE ;)"
This effectively reverts commit 8ee74a91ac30 ("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>
Diffstat (limited to 'fs/proc')
-rw-r--r--fs/proc/base.c2
1 files changed, 1 insertions, 1 deletions
diff --git a/fs/proc/base.c b/fs/proc/base.c
index 45f6bf68fff3..f1e1927ccd48 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -821,7 +821,7 @@ static ssize_t mem_rw(struct file *file, char __user *buf,
if (!mmget_not_zero(mm))
goto free;
- flags = write ? FOLL_WRITE : 0;
+ flags = FOLL_FORCE | (write ? FOLL_WRITE : 0);
while (count > 0) {
int this_len = min_t(int, count, PAGE_SIZE);