Michael Ellerman [off-list ref] writes:
Breno Leitao [off-list ref] writes:
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On Tue, Mar 12, 2024 at 08:17:42AM +0000, Christophe Leroy wrote:
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+Nathan as this is RTAS related.
Thanks!
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Le 21/08/2018 à 20:42, Breno Leitao a écrit :
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The rtas syscall reads a value from a user-provided structure and uses it
to index an array, being a possible area for a potential spectre v1 attack.
This is the code that exposes this problem.
args.rets = &args.args[nargs];
The nargs is an user provided value, and the below code is an example where
the 'nargs' value would be set to XX.
struct rtas_args ra;
ra.nargs = htobe32(XX);
syscall(__NR_rtas, &ra);
This patch has been hanging around in patchwork since 2018 and doesn't
apply anymore. Is it still relevant ? If so, can you rebase et resubmit ?
This seems to be important, since nargs is a user-provided value. I can
submit it if the maintainers are willing to accept. I do not want to
spend my time if no one is willing to review it.
My memory is that I didn't think it was actually a problem, because all
we do is memset args.rets to zero.
This is also my initial reaction to this. I suppose if the memset()
implementation performs some validation of the destination buffer
contents (comparing to a known poison value or something) that could
load the CPU cache then there is a more plausible issue?
Anyway we should probably just fix it to be safe and keep the static
checkers happy.
Here is the relevant passage in its current state:
if (copy_from_user(&args, uargs, 3 * sizeof(u32)) != 0)
return -EFAULT;
nargs = be32_to_cpu(args.nargs);
nret = be32_to_cpu(args.nret);
token = be32_to_cpu(args.token);
if (nargs >= ARRAY_SIZE(args.args)
|| nret > ARRAY_SIZE(args.args)
|| nargs + nret > ARRAY_SIZE(args.args))
return -EINVAL;
/* Copy in args. */
if (copy_from_user(args.args, uargs->args,
nargs * sizeof(rtas_arg_t)) != 0)
return -EFAULT;
/*
* If this token doesn't correspond to a function the kernel
* understands, you're not allowed to call it.
*/
func = rtas_token_to_function_untrusted(token);
if (!func)
return -EINVAL;
args.rets = &args.args[nargs];
memset(args.rets, 0, nret * sizeof(rtas_arg_t));
Some questions:
1. The patch sanitizes 'nargs' immediately before the call to memset(),
but shouldn't that happen before 'nargs' is used as an input to
copy_from_user()?
2. If 'nargs' needs this treatment, then why wouldn't the user-supplied
'nret' and 'token' need them as well? 'nret' is used to index the
same array as 'nargs'. And at least conceptually, 'token' is used to
index a data structure (xarray) with array-like semantics (to be
fair, this is a relatively recent development and was not the case
when this change was submitted).