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b=AC+YzHU8leL8i+EYVMpSqsmzKiAwgAUWMET8Zqi8/r/s8NWsptyd0RLJpkHu/dkeg yE5o/jIS3/0wrBOJqH8nAI31EIgqyKG/Buryxe6dxC8NZ6ldLsBi50O/YR1m6wLYrZ jkL5JMORWiTOkihdNedepP5CwwgakNGzN1jONoiU= Date: Tue, 1 Oct 2024 15:04:38 -0700 From: Andrew Morton To: Patrick Roy Cc: , , , , , Subject: Re: [PATCH] secretmem: disable memfd_secret() if arch cannot set direct map Message-Id: <20241001150438.017b7bb4cd1baceb53a764bf@linux-foundation.org> In-Reply-To: <20241001080056.784735-1-roypat@amazon.co.uk> References: <20241001080056.784735-1-roypat@amazon.co.uk> X-Mailer: Sylpheed 3.8.0beta1 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 706E7140007 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: euth13rqja46essyujwo4yhwua3kyxaa X-HE-Tag: 1727820280-542664 X-HE-Meta: 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 /ae0i355 UaQ/9HcUfd4y/exRXtMiYciV7jWqyefm0tqARPqRTelfaJ0/I3PH67xY2LcrqvmLVH77ixrk9nHU6ouA1ShFTbSXfhXw1yb8Ihrx92rb/bpwDVCC/MRS6V4zfgw5vejniXcgpEo+5LHU7wKNla1LtcApmAaszHRioQYMLgYmSqTplZTIujceRJkvpKR8cgLhTqEV2aq8SXHVMHqsN3iVJmTDYTlv47ek1Qr68MQpNXZ9VSZONneyA1Czxki8DpzPqwd9IONuI32pCrRzGGTErs0vcSfd0AnS4kwaK8fjCD2nGLyoAf4m18DwWO2EeaQFtETb6XUQsMdjmjqofvklCR558Ag== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, 1 Oct 2024 09:00:41 +0100 Patrick Roy wrote: > Return -ENOSYS from memfd_secret() syscall if !can_set_direct_map(). > This is the case for example on some arm64 configurations, where marking > 4k PTEs in the direct map not present can only be done if the direct map > is set up at 4k granularity in the first place (as ARM's > break-before-make semantics do not easily allow breaking apart > large/gigantic pages). > > More precisely, on arm64 systems with !can_set_direct_map(), > set_direct_map_invalid_noflush() is a no-op, however it returns success > (0) instead of an error. This means that memfd_secret will seemingly > "work" (e.g. syscall succeeds, you can mmap the fd and fault in pages), > but it does not actually achieve its goal of removing its memory from > the direct map. > > Note that with this patch, memfd_secret() will start erroring on systems > where can_set_direct_map() returns false (arm64 with > CONFIG_RODATA_FULL_DEFAULT_ENABLED=n, CONFIG_DEBUG_PAGEALLOC=n and > CONFIG_KFENCE=n), but that still seems better than the current silent > failure. Since CONFIG_RODATA_FULL_DEFAULT_ENABLED defaults to 'y', most > arm64 systems actually have a working memfd_secret() and aren't be > affected. > > >From going through the iterations of the original memfd_secret patch > series, it seems that disabling the syscall in these scenarios was the > intended behavior [1] (preferred over having > set_direct_map_invalid_noflush return an error as that would result in > SIGBUSes at page-fault time), however the check for it got dropped > between v16 [2] and v17 [3], when secretmem moved away from CMA > allocations. > > [1]: https://lore.kernel.org/lkml/20201124164930.GK8537@kernel.org/ > [2]: https://lore.kernel.org/lkml/20210121122723.3446-11-rppt@kernel.org/#t > [3]: https://lore.kernel.org/lkml/20201125092208.12544-10-rppt@kernel.org/ Thanks. > Fixes: 1507f51255c9 ("mm: introduce memfd_secret system call to create "secret" memory areas") So I'm thinking this fix should be backported into kernels which contain 1507f51255c9, agree?