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b87c8c4aca
Currently, vec_set_vector_length() can manipulate a task into an invalid
state as a result of a prctl/ptrace syscall which changes the SVE/SME
vector length, resulting in several problems:
(1) When changing the SVE vector length, if the task initially has
PSTATE.ZA==1, and sve_alloc() fails to allocate memory, the task
will be left with PSTATE.ZA==1 and sve_state==NULL. This is not a
legitimate state, and could result in a subsequent null pointer
dereference.
(2) When changing the SVE vector length, if the task initially has
PSTATE.SM==1, the task will be left with PSTATE.SM==1 and
fp_type==FP_STATE_FPSIMD. Streaming mode state always needs to be
saved in SVE format, so this is not a legitimate state.
Attempting to restore this state may cause a task to erroneously
inherit stale streaming mode predicate registers and FFR contents,
behaving non-deterministically and potentially leaving information
from another task.
While in this state, reads of the NT_ARM_SSVE regset will indicate
that the registers are not stored in SVE format. For the NT_ARM_SSVE
regset specifically, debuggers interpret this as meaning that
PSTATE.SM==0.
(3) When changing the SME vector length, if the task initially has
PSTATE.SM==1, the lower 128 bits of task's streaming mode vector
state will be migrated to non-streaming mode, rather than these bits
being zeroed as is usually the case for changes to PSTATE.SM.
To fix the first issue, we can eagerly allocate the new sve_state and
sme_state before modifying the task. This makes it possible to handle
memory allocation failure without modifying the task state at all, and
removes the need to clear TIF_SVE and TIF_SME.
To fix the second issue, we either need to clear PSTATE.SM or not change
the saved fp_type. Given we're going to eagerly allocate sve_state and
sme_state, the simplest option is to preserve PSTATE.SM and the saves
fp_type, and consistently truncate the SVE state. This ensures that the
task always stays in a valid state, and by virtue of not exiting
streaming mode, this also sidesteps the third issue.
I believe these changes should not be problematic for realistic usage:
* When the SVE/SME vector length is changed via prctl(), syscall entry
will have cleared PSTATE.SM. Unless the task's state has been
manipulated via ptrace after entry, the task will have PSTATE.SM==0.
* When the SVE/SME vector length is changed via a write to the
NT_ARM_SVE or NT_ARM_SSVE regsets, PSTATE.SM will be forced
immediately after the length change, and new vector state will be
copied from userspace.
* When the SME vector length is changed via a write to the NT_ARM_ZA
regset, the (S)SVE state is clobbered today, so anyone who cares about
the specific state would need to install this after writing to the
NT_ARM_ZA regset.
As we need to free the old SVE state while TIF_SVE may still be set, we
cannot use sve_free(), and using kfree() directly makes it clear that
the free pairs with the subsequent assignment. As this leaves sve_free()
unused, I've removed the existing sve_free() and renamed __sve_free() to
mirror sme_free().
Fixes:
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||
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.