On 2024-05-23, Jeff Xu <jeffxu@xxxxxxxxxx> wrote: > On Thu, May 23, 2024 at 1:24 AM David Rheinsberg <david@xxxxxxxxxxxx> wrote: > > > > Hi > > > > On Thu, May 23, 2024, at 4:25 AM, Barnabás Pőcze wrote: > > > 2024. május 23., csütörtök 1:23 keltezéssel, Andrew Morton > > > <akpm@xxxxxxxxxxxxxxxxxxxx> írta: > > >> It's a change to a userspace API, yes? Please let's have a detailed > > >> description of why this is OK. Why it won't affect any existing users. > > > > > > Yes, it is a uAPI change. To trigger user visible change, a program has to > > > > > > - create a memfd > > > - with MFD_NOEXEC_SEAL, > > > - without MFD_ALLOW_SEALING; > > > - try to add seals / check the seals. > > > > > > This change in essence reverts the kernel's behaviour to that of Linux > > > <6.3, where > > > only `MFD_ALLOW_SEALING` enabled sealing. If a program works correctly > > > on those > > > kernels, it will likely work correctly after this change. > > > > > > I have looked through Debian Code Search and GitHub, searching for > > > `MFD_NOEXEC_SEAL`. > > > And I could find only a single breakage that this change would case: > > > dbus-broker > > > has its own memfd_create() wrapper that is aware of this implicit > > > `MFD_ALLOW_SEALING` > > > behaviour[0], and tries to work around it. This workaround will break. > > > Luckily, > > > however, as far as I could tell this only affects the test suite of > > > dbus-broker, > > > not its normal operations, so I believe it should be fine. I have > > > prepared a PR > > > with a fix[1]. > > > > We asked for exactly this fix before, so I very much support this. Our test-suite in `dbus-broker` merely verifies what the current kernel behavior is (just like the kernel selftests). I am certainly ok if the kernel breaks it. I will gladly adapt the test-suite. > > > > Previous discussion was in: > > > > [PATCH] memfd: support MFD_NOEXEC alongside MFD_EXEC > > https://lore.kernel.org/lkml/20230714114753.170814-1-david@xxxxxxxxxxxx/ > > > > Note that this fix is particularly important in combination with `vm.memfd_noexec=2`, since this breaks existing user-space by enabling sealing on all memfds unconditionally. I also encourage backporting to stable kernels. > > > Also with vm.memfd_noexec=1. > I think that problem must be addressed either with this patch, or with > a new flag. > > Regarding vm.memfd_noexec, on another topic. > I think in addition to vm.memfd_noexec = 1 and 2, there still could > be another state: 3 > > =0. Do nothing. > =1. This will add MFD_NOEXEC_SEAL if application didn't set EXEC or > MFD_NOEXE_SEAL (to help with the migration) > =2: This will reject all calls without MFD_NOEXEC_SEAL (the whole > system doesn't allow executable memfd) > =3: Application must set MFD_EXEC or MFD_NOEXEC_SEAL explicitly, or > else it will be rejected. > > 3 is useful because it lets applications choose what to use, and > forces applications to migrate to new semantics (this is what 2 did > before 9876cfe8). > The caveat is 3 is less restrictive than 2, so must document it clearly. As discussed at the time, "you must use this flag" is not a useful setting for a general purpose operating system because it explicitly disables backwards compatibility (breaking any application that was written in the past 10 years!) for no reason other than "new is better". As I suggested when we fixed the semantics of vm.memfd_noexec, if you really want to block a particular flag from not being set, seccomp lets you do this incredibly easily without acting as a footgun for admins. Yes, vm.memfd_noexec can break programs that use executable memfds, but that is the point of the sysctl -- making vm.memfd_noexec break programs that don't use executable memfds (they are only guilty of being written before mid-2023) is not useful. In addition, making 3 less restrictive than 2 would make the original restriction mechanism useless. A malicious process could raise the setting to 3 and disable the "protection" (as discussed before, I really don't understand the threat model here, but making it possible to disable easily is pretty clearly). You could change the policy, but now you're adding more complexity for a feature that IMO doesn't really make sense in the first place. > -Jeff > > > Reviewed-by: David Rheinsberg <david@xxxxxxxxxxxx> > > > > Thanks > > David -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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