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. -Jeff > Reviewed-by: David Rheinsberg <david@xxxxxxxxxxxx> > > Thanks > David
