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Currently, all filesystems that support fsverity (ext4, f2fs, and btrfs)
cache the Merkle tree in the pagecache at a 64K aligned offset after the
end of the file data. This offset needs to be a multiple of the page
size, which is guaranteed only when the page size is 64K or smaller.
64K was chosen to be the "largest reasonable page size". But it isn't
the largest *possible* page size: the hexagon and powerpc ports of Linux
support 256K pages, though that configuration is rarely used.
For now, just disable support for FS_VERITY in these odd configurations
to ensure it isn't used in cases where it would have incorrect behavior.
Fixes: 671e67b47e ("fs-verity: add Kconfig and the helper functions for hashing")
Reported-by: Christoph Hellwig <hch@lst.de>
Closes: https://lore.kernel.org/r/20260119063349.GA643@lst.de
Reviewed-by: Theodore Ts'o <tytso@mit.edu>
Link: https://lore.kernel.org/r/20260221204525.30426-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
49 lines
2 KiB
Text
49 lines
2 KiB
Text
# SPDX-License-Identifier: GPL-2.0
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config FS_VERITY
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bool "FS Verity (read-only file-based authenticity protection)"
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# Filesystems cache the Merkle tree at a 64K aligned offset in the
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# pagecache. That approach assumes the page size is at most 64K.
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depends on PAGE_SHIFT <= 16
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select CRYPTO_HASH_INFO
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select CRYPTO_LIB_SHA256
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select CRYPTO_LIB_SHA512
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help
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This option enables fs-verity. fs-verity is the dm-verity
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mechanism implemented at the file level. On supported
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filesystems (currently ext4, f2fs, and btrfs), userspace can
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use an ioctl to enable verity for a file, which causes the
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filesystem to build a Merkle tree for the file. The filesystem
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will then transparently verify any data read from the file
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against the Merkle tree. The file is also made read-only.
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This serves as an integrity check, but the availability of the
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Merkle tree root hash also allows efficiently supporting
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various use cases where normally the whole file would need to
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be hashed at once, such as: (a) auditing (logging the file's
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hash), or (b) authenticity verification (comparing the hash
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against a known good value, e.g. from a digital signature).
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fs-verity is especially useful on large files where not all
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the contents may actually be needed. Also, fs-verity verifies
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data each time it is paged back in, which provides better
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protection against malicious disks vs. an ahead-of-time hash.
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If unsure, say N.
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config FS_VERITY_BUILTIN_SIGNATURES
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bool "FS Verity builtin signature support"
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depends on FS_VERITY
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select SYSTEM_DATA_VERIFICATION
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help
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This option adds support for in-kernel verification of
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fs-verity builtin signatures.
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Please take great care before using this feature. It is not
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the only way to do signatures with fs-verity, and the
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alternatives (such as userspace signature verification, and
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IMA appraisal) can be much better. For details about the
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limitations of this feature, see
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Documentation/filesystems/fsverity.rst.
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If unsure, say N.
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