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SubjectRe: [PATCH 1/3] x86/entry: Clear extra registers beyond syscall arguments for 64bit kernels

* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> [...]
>
> But as the commit message says, the system call argument registers are
> also likely to be aggressively clobbered unless used, since the low
> registers are preferred for code generation (smaller code, and many of
> them are special anyway in various ways and have forced uses for
> shifts, function arguments, or just are special in general like %rax).
>
> So the actual argument registers tend to not be an issue anyway.

Btw., to underline these arguments, here's some statistical data about actual
register usage the x86 kernel.

I picked the latest upstream kernel and did a statistical analysis of the
disassembly of an 'allyesconfig' 64-bit build.

Here is the histogram of GP register usage, ordered by a calculated "average
per-function usage ratio" (last column):

# nr of =y .config options: 9553
# nr of functions: 249340
# nr of instructions: 20223765
# nr of register uses: 33413619

register | # of uses | avg uses per fn
--------------------------------------
%r11 | 21564 | 0.1
%r10 | 65499 | 0.3
%r9 | 162040 | 0.6
%r8 | 292779 | 1.2
%rcx | 860528 | 3.5
%r15 | 1414816 | 5.7
%r14 | 1597952 | 6.4
%rsi | 1636660 | 6.6
%rdx | 1798109 | 7.2
%r13 | 1829557 | 7.3
%r12 | 2301476 | 9.2
%rbp | 3156682 | 12.7
%rbx | 4451880 | 17.9
%rdi | 4747951 | 19.0
%rax | 5370191 | 21.5

Here is the same histogram for a distro kernel (Fedora) config based build, with
all =m modules changed to =y and thus built into the vmlinux for easier analysis:

# nr of =y .config options: 4871
# nr of functions: 190477
# nr of instructions: 10329411
# nr of register uses: 16907185

register | # of uses | avg uses per fn
--------------------------------------
%r11 | 64135 | 0.3
%r10 | 113366 | 0.6
%r9 | 196269 | 1.0
%r8 | 314812 | 1.7
%r15 | 404789 | 2.1
%r14 | 461266 | 2.4
%r13 | 569654 | 3.0
%r12 | 763973 | 4.0
%rcx | 920477 | 4.8
%rbp | 1161700 | 6.1
%rsi | 1257150 | 6.6
%rdi | 1625617 | 8.5
%rdx | 1667365 | 8.8
%rbx | 1739660 | 9.1
%rax | 3826187 | 20.1

Finally here's the histogram of a 'defconfig' build - which should be
representative of 'device specific kernel builds':

# nr of =y .config options: 1255
# nr of functions: 45490
# nr of instructions: 1963956
# nr of register uses: 3183680

register | # of uses | avg uses per fn
--------------------------------------
%r11 | 11608 | 0.3
%r10 | 23398 | 0.5
%r9 | 37431 | 0.8
%r8 | 56140 | 1.2
%r15 | 77468 | 1.7
%r14 | 89285 | 2.0
%r13 | 111665 | 2.5
%r12 | 151977 | 3.3
%rcx | 166425 | 3.7
%rsi | 226536 | 5.0
%rbp | 238286 | 5.2
%rdi | 306709 | 6.7
%rdx | 313569 | 6.9
%rbx | 349496 | 7.7
%rax | 728036 | 16.0

(Note the various caveats listed further below.)

These three builds I believe provide representative members of a wide spectrum of
kernel options used in practice: from everything-enabled, through distro-enabled
to device-specific minimal kernels.

There's a consistent pattern in these histograms: the least used registers are
R11, R10, R9 and R8. Registers R12-R15 are used almost as frequently as some of
the GP registers (!).

In practice R11-R10 is probably the most vulnerable ones to attack: their use is
at least 1-2 orders of magnitude less common than that of the more common general
purpose registers.

So I submit that we should probably extend the register clearing/sanitization to
R10 and R11 as well, because while they are technically caller-saved and freely
clobberable, in practice they don't get clobbered all that often and there might
be various code paths into complex system calls where these R10/R11 values survive
just fine and can be used in Spectre gadgets.

Thanks,

Ingo

P.S.:

List of caveats/notes:

Note #1:
I collapsed all 32-bit register users which zero-extend by default.
I did not collapse 8 and 16 bit uses as they don't automatically clobber the
higher bits. )

Note #2:
This histogram does not make a distinction between read and write uses of
registers.

Note #3:
I did not include implicit register clobbering, only those registers that are
explicitly listed in the disassembly. In the overwhelming majority of cases the
affected registers are listed though, so the real numbers should be very close
though.

Note #4:
The 'avg uses per fn' number is over-estimates the real uses per function,
because I counted total number of uses, not rounded down to a per function
register usage heat-map. I believe this does not change the _ordering_ of the
register usage histograms, so it's a valid simplification.

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