On 7/28/26 10:55 AM, Jiacheng Yu wrote:
> param_set_charp() stores charp parameters in allocated memory after slab is
> available, and releases the previous value when the parameter is updated.
>
> The previous value is released before the replacement allocation succeeds.
> If kmalloc_parameter() fails, the setter returns -ENOMEM with the parameter
> left as NULL.
>
> Failing zswap's compressor update before zswap is initialized can later
> trigger:
>
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> RIP: 0010:strcmp+0x10/0x30
> Call Trace:
> zswap_setup+0x3b1/0x490
> zswap_enabled_param_set+0x5b/0xa0
> param_attr_store+0x93/0xe0
> module_attr_store+0x1c/0x30
> kernfs_fop_write_iter+0x116/0x1f0
>
> Allocate and copy the replacement first, then replace the parameter value
> only after allocation succeeds.
>
> Fixes: e180a6b7759a ("param: fix charp parameters set via sysfs")
> Cc: [email protected]
> Signed-off-by: Jiacheng Yu <[email protected]>
It makes sense to me for param_set_charp() to have commit-or-rollback
semantics. The set callbacks of other standard parameters behave this
way, with the exception of array parameters.
> ---
> kernel/params.c | 14 ++++++++------
> 1 file changed, 8 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/params.c b/kernel/params.c
> index a668863a4bb6..e4f2b71dde1e 100644
> --- a/kernel/params.c
> +++ b/kernel/params.c
> @@ -261,6 +261,7 @@ EXPORT_SYMBOL_GPL(param_set_uint_minmax);
>
> int param_set_charp(const char *val, const struct kernel_param *kp)
> {
> + char *tmp;
> size_t len, maxlen = 1024;
>
> len = strnlen(val, maxlen + 1);
> @@ -269,19 +270,20 @@ int param_set_charp(const char *val, const struct
> kernel_param *kp)
> return -ENOSPC;
> }
>
> - maybe_kfree_parameter(*(char **)kp->arg);
> -
> /*
> * This is a hack. We can't kmalloc() in early boot, and we
> * don't need to; this mangled commandline is preserved.
> */
> if (slab_is_available()) {
> - *(char **)kp->arg = kmalloc_parameter(len + 1);
> - if (!*(char **)kp->arg)
> + tmp = kmalloc_parameter(len + 1);
> + if (!tmp)
> return -ENOMEM;
> - strcpy(*(char **)kp->arg, val);
> + strscpy(tmp, val, len + 1);
What's wrong with the plain strcpy() here?
> } else
> - *(const char **)kp->arg = val;
> + tmp = (char *)val;
> +
> + maybe_kfree_parameter(*(char **)kp->arg);
> + *(char **)kp->arg = tmp;
Sashiko reports [1] that there is a pre-existing use-after-free window
between freeing the old parameter and updating kp->arg. However, this
issue doesn't appear to be valid because any concurrent access to
a writable charp parameter should be protected by kernel_param_lock().
This is documented include/linux/moduleparam.h [2].
>
> return 0;
> }
[1]
https://lore.kernel.org/linux-modules/[email protected]/
[2]
https://github.com/torvalds/linux/blob/v7.2-rc5/include/linux/moduleparam.h#L124
--
Thanks,
Petr
