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vmm: Fix UB in AMX support checks

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Rust's aliasing rules do not permit mutating a `usize` through a shared
reference. Unsafe blocks and FFI are no exception.

This PR fixes such a violation detected in the AMX support checks by
passing a raw mutable pointer instead of a shared reference in the
syscall.

Signed-off-by: Oliver Anderson <oliver.anderson@cyberus-technology.de>
On-behalf-of: SAP <oliver.anderson@sap.com>
This commit is contained in:
Oliver Anderson
2025-09-26 12:29:43 +02:00
committed by Rob Bradford
parent bb713e8c01
commit 4c16285dde
1 changed files with 9 additions and 4 deletions
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13
vmm/src/cpu.rs
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@@ -730,11 +730,16 @@ impl CpuManager {
if amx_tile != 0 {
return Err(Error::AmxEnable(anyhow!("Guest AMX usage not supported")));
} else {
let mask: usize = 0;
// SAFETY: the mask being modified (not marked mutable as it is
// modified in unsafe only which is permitted) isn't in use elsewhere.
let mut mask: usize = 0;
// SAFETY: Syscall with valid parameters. We use a raw mutable pointer to
// the `mask` place in order to ensure that we do not violate Rust's
// aliasing rules.
let result = unsafe {
libc::syscall(libc::SYS_arch_prctl, ARCH_GET_XCOMP_GUEST_PERM, &mask)
libc::syscall(
libc::SYS_arch_prctl,
ARCH_GET_XCOMP_GUEST_PERM,
&raw mut mask,
)
};
if result != 0 || (mask & XFEATURE_XTILEDATA_MASK) != XFEATURE_XTILEDATA_MASK {
return Err(Error::AmxEnable(anyhow!("Guest AMX usage not supported")));