Messages in this thread | | | Date | Tue, 23 May 2023 13:53:09 +0300 | From | Alexey Izbyshev <> | Subject | Re: [PATCH v2 3/5] mm: Make PR_MDWE_REFUSE_EXEC_GAIN an unsigned long |
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On 2023-05-23 12:12, David Hildenbrand wrote: > On 22.05.23 20:58, Alexey Izbyshev wrote: >> On 2023-05-22 19:22, David Hildenbrand wrote: >>> On 22.05.23 12:35, Alexey Izbyshev wrote: >>>> On 2023-05-22 11:55, David Hildenbrand wrote: >>>>> On 17.05.23 17:03, Florent Revest wrote: >>>>>> Alexey pointed out that defining a prctl flag as an int is a >>>>>> footgun >>>>>> because, under some circumstances, when used as a flag to prctl, >>>>>> it >>>>>> can >>>>>> be casted to long with garbage upper bits which would result in >>>>>> unexpected behaviors. >>>>>> >>>>>> This patch changes the constant to a UL to eliminate these >>>>>> possibilities. >>>>>> >>>>>> Signed-off-by: Florent Revest <revest@chromium.org> >>>>>> Suggested-by: Alexey Izbyshev <izbyshev@ispras.ru> >>>>>> --- >>>>>> include/uapi/linux/prctl.h | 2 +- >>>>>> tools/include/uapi/linux/prctl.h | 2 +- >>>>>> 2 files changed, 2 insertions(+), 2 deletions(-) >>>>>> >>>>>> diff --git a/include/uapi/linux/prctl.h >>>>>> b/include/uapi/linux/prctl.h >>>>>> index f23d9a16507f..6e9af6cbc950 100644 >>>>>> --- a/include/uapi/linux/prctl.h >>>>>> +++ b/include/uapi/linux/prctl.h >>>>>> @@ -283,7 +283,7 @@ struct prctl_mm_map { >>>>>> /* Memory deny write / execute */ >>>>>> #define PR_SET_MDWE 65 >>>>>> -# define PR_MDWE_REFUSE_EXEC_GAIN 1 >>>>>> +# define PR_MDWE_REFUSE_EXEC_GAIN (1UL << 0) >>>>>> #define PR_GET_MDWE 66 >>>>>> diff --git a/tools/include/uapi/linux/prctl.h >>>>>> b/tools/include/uapi/linux/prctl.h >>>>>> index 759b3f53e53f..6e6563e97fef 100644 >>>>>> --- a/tools/include/uapi/linux/prctl.h >>>>>> +++ b/tools/include/uapi/linux/prctl.h >>>>>> @@ -283,7 +283,7 @@ struct prctl_mm_map { >>>>>> /* Memory deny write / execute */ >>>>>> #define PR_SET_MDWE 65 >>>>>> -# define PR_MDWE_REFUSE_EXEC_GAIN 1 >>>>>> +# define PR_MDWE_REFUSE_EXEC_GAIN (1UL << 0) >>>>>> #define PR_GET_MDWE 66 >>>>>> >>>>> >>>>> Both are changing existing uapi, so you'll already have existing >>>>> user >>>>> space using the old values, that your kernel code has to deal with >>>>> no? >>>> >>>> I'm the one who suggested this change, so I feel the need to >>>> clarify. >>>> >>>> For any existing 64-bit user space code using the kernel and the >>>> uapi >>>> headers before this patch and doing the wrong prctl(PR_SET_MDWE, >>>> PR_MDWE_REFUSE_EXEC_GAIN) call instead of the correct >>>> prctl(PR_SET_MDWE, >>>> (unsigned long)PR_MDWE_REFUSE_EXEC_GAIN), there are two >>>> possibilities >>>> when prctl() implementation extracts the second argument via >>>> va_arg(op, >>>> unsigned long): >>>> >>>> * It gets lucky, and the upper 32 bits of the argument are zero. The >>>> call does what is expected by the user. >>>> >>>> * The upper 32 bits are non-zero junk. The flags argument is >>>> rejected >>>> by >>>> the kernel, and the call fails with EINVAL (unexpectedly for the >>>> user). >>>> >>>> This change is intended to affect only the second case, and only >>>> after >>>> the program is recompiled with the new uapi headers. The currently >>>> wrong, but naturally-looking prctl(PR_SET_MDWE, >>>> PR_MDWE_REFUSE_EXEC_GAIN) call becomes correct. >>>> >>>> The kernel ABI is unaffected by this change, since it has been >>>> defined >>>> in terms of unsigned long from the start. >>> >>> The thing I'm concerned about is the following: old user space (that >>> would fail) on new kernel where we define some upper 32bit to >>> actually >>> have a meaning (where it would succeed with wrong semantics). >>> >>> IOW, can we ever really "use" these upper 32bit, or should we instead >>> only consume the lower 32bit in the kernel and effectively ignore the >>> upper 32bit? >>> >> I see, thanks. But I think this question is mostly independent from >> this >> patch. The patch removes a footgun, but it doesn't change the flags >> check in the kernel, and nothing stops the user from doing >> >> int flags = PR_MDWE_REFUSE_EXEC_GAIN; >> prctl(PR_SET_MDWE, flags); >> >> So we have to decide whether to ignore the upper 32 bits or not even >> if >> this patch is not applied (actually *had to* when PR_SET_MDWE op was >> being added). > > Well, an alternative to this patch would be to say "well, for this > prctl we ignore any upper 32bit. Then, this change would not be > needed. Yes, I also don't like that :) > > Bu unrelated, I looked at some other random prctl. > > #define SUID_DUMP_USER 1 > > And in kernel/sys.c: > > case PR_SET_DUMPABLE: > if (arg2 != SUID_DUMP_DISABLE && arg2 != SUID_DUMP_USER) > ... > > Wouldn't that also suffer from the same issue, or how is this > different? > Yes, it is the same issue, so e.g. prctl(PR_SET_DUMPABLE, SUID_DUMP_DISABLE ) may wrongly fail with EINVAL on 64-bit targets.
> Also, how is passing "0"s to e.g., PR_GET_THP_DISABLE reliable? We > need arg2 -> arg5 to be 0. But wouldn't the following also just pass a > 0 "int" ? > > prctl(PR_GET_THP_DISABLE, 0, 0, 0, 0) > Yes, this is not reliable on 64-bit targets too. The simplest fix is to use "0L", as done in MDWE self-tests (but many other tests get this wrong).
Florent also expressed surprise[1] that we don't see a lot of failures due to such issues, and I tried to provide some reasons. To elaborate on the x86-64 thing, for prctl(PR_SET_DUMPABLE, 0) the compiler will likely generate "xorl %esi, %esi" to pass zero, but this instruction will also clear the upper 32 bits of %rsi, so the problem is masked (and I believe CPU vendors are motivated to do such zeroing to reduce false dependencies). But this zeroing is not required by the ABI, so in a more complex situation junk might get through.
Real-world examples of very similar breakage in variadic functions involving NULL sentinels are mentioned in [2] (the musl bug report is [3]). In short, musl defined NULL as plain 0 for C++, so when people do e.g. execl("/bin/true", "true", NULL), junk might prevent detection of the sentinel in execl() impl. (Though if the sentinel is passed via stack because there are a lot of preceding arguments, the breakage becomes more apparent because auto-zeroing of registers doesn't come into play anymore.)
> > I'm easily confused by such (va_args) things, so sorry for the dummy > questions.
This stuff *is* confusing, and note that Linux man pages don't even tell that prctl() is actually declared as a variadic function (and for ptrace() this is mentioned only in the notes, but not in its signature).
Thanks, Alexey
[1] https://lore.kernel.org/lkml/3a38319a3b241e578729ffa5484ad24b@ispras.ru [2] https://ewontfix.com/11 [3] https://www.openwall.com/lists/musl/2013/01/09/1
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