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If the system exposes memory regions with the EFI_MORE_RELIABLE
attribute, it is implied that it is intended to be used for allocations
that are relatively important, such as the kernel's static image.
Since efi_random_alloc() is mostly (only) used for allocating space for
the kernel image, let's update it to take this into account, and
disregard all memory without the EFI_MORE_RELIABLE attribute if there is
sufficient memory available that does have this attribute.
Note that this change only affects booting with randomization enabled.
In other cases, the EFI stub runs the kernel image in place unless its
placement is unsuitable for some reason (i.e., misaligned, or its BSS
overlaps with another allocation), and it is left to the bootloader to
ensure that the kernel was loaded into EFI_MORE_RELIABLE memory if this
is desired.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
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The EFI stub random allocator used for kaslr on arm64 has a subtle
bug. In function get_entry_num_slots() which counts the number of
possible allocation "slots" for the image in a given chunk of free
EFI memory, "last_slot" can become negative if the chunk is smaller
than the requested allocation size.
The test "if (first_slot > last_slot)" doesn't catch it because
both first_slot and last_slot are unsigned.
I chose not to make them signed to avoid problems if this is ever
used on architectures where there are meaningful addresses with the
top bit set. Instead, fix it with an additional test against the
allocation size.
This can cause a boot failure in addition to a loss of randomisation
due to another bug in the arm64 stub fixed separately.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Fixes: 2ddbfc81eac8 ("efi: stub: add implementation of efi_random_alloc()")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
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The implementation of efi_random_alloc() arbitrarily truncates the
provided random seed to 16 bits, which limits the granularity of the
randomly chosen allocation offset in memory. This is currently only
an issue if the size of physical memory exceeds 128 GB, but going
forward, we will reduce the allocation alignment to 64 KB, and this
means we need to increase the granularity to ensure that the random
memory allocations are distributed evenly.
We will need to switch to 64-bit arithmetic for the multiplication,
but this does not result in 64-bit integer intrinsic calls on ARM or
on i386.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
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The EFI stub uses a per-architecture #define for the minimum base
and size alignment of page allocations, which is set to 4 KB for
all architecures except arm64, which uses 64 KB, to ensure that
allocations can always be (un)mapped efficiently, regardless of
the page size used by the kernel proper, which could be a kexec'ee
The API wrappers around page based allocations assume that this
alignment is always taken into account, and so efi_free() will
also round up its size argument to EFI_ALLOC_ALIGN.
Currently, efi_random_alloc() does not honour this alignment for
the allocated size, and so freeing such an allocation may result
in unrelated memory to be freed, potentially leading to issues
after boot. So let's round up size in efi_random_alloc() as well.
Fixes: 2ddbfc81eac84a29 ("efi: stub: add implementation of efi_random_alloc()")
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
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efi_random_alloc() is only used on arm64, but as it shares a source
file with efi_random_get_seed(), the latter will pull in the former
on other architectures as well.
Let's take advantage of the fact that libstub is a static library,
and so the linker will only incorporate objects that are needed to
satisfy dependencies in other objects. This means we can move the
random alloc code to a separate source file that gets built
unconditionally, but only used when needed.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
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