CVE-2024-57874: arm64: ptrace: fix partial SETREGSET for NT_ARM_TAGGED_ADDR_CTRL

In the Linux kernel, the following vulnerability has been resolved:

arm64: ptrace: fix partial SETREGSET for NTARMTAGGEDADDRCTRL

Currently taggedaddrctrlset() doesn't initialize the temporary 'ctrl' variable, and a SETREGSET call with a length of zero will leave this uninitialized. Consequently taggedaddrctrlset() will consume an arbitrary value, potentially leaking up to 64 bits of memory from the kernel stack. The read is limited to a specific slot on the stack, and the issue does not provide a write mechanism.

As settaggedaddrctrl() only accepts values where bits [63:4] zero and rejects other values, a partial SETREGSET attempt will randomly succeed or fail depending on the value of the uninitialized value, and the exposure is significantly limited.

Fix this by initializing the temporary value before copying the regset from userspace, as for other regsets (e.g. NTPRSTATUS, NTPRFPREG, NTARMSYSTEMCALL). In the case of a zero-length write, the existing value of the tagged address ctrl will be retained.

The NTARMTAGGEDADDRCTRL regset is only visible in the useraarch64view used by a native AArch64 task to manipulate another native AArch64 task. As gettaggedaddrctrl() only returns an error value when called for a compat task, taggedaddrctrlget() and taggedaddrctrlset() should never observe an error value from gettaggedaddrctrl(). Add a WARNONONCE() to both to indicate that such an error would be unexpected, and error handlnig is not missing in either case.