Re: [PATCH] lockdown: Fix kexec lockdown bypass with ima policy

[John Haxby]

> On 19 Jul 2022, at 18:16, Eric Snowberg <eric.snowberg@xxxxxxxxxx> wrote:
> 
> The lockdown LSM is primarily used in conjunction with UEFI Secure Boot.
> This LSM may also be used on machines without UEFI.  It can also be enabled
> when UEFI Secure Boot is disabled. One of lockdown's features is to prevent
> kexec from loading untrusted kernels. Lockdown can be enabled through a
> bootparam or after the kernel has booted through securityfs.
> 
> If IMA appraisal is used with the "ima_appraise=log" boot param,
> lockdown can be defeated with kexec on any machine when Secure Boot is
> disabled or unavailable. IMA prevents setting "ima_appraise=log"
> from the boot param when Secure Boot is enabled, but this does not cover
> cases where lockdown is used without Secure Boot.
> 
> To defeat lockdown, boot without Secure Boot and add ima_appraise=log
> to the kernel command line; then:
> 
> $ echo "integrity" > /sys/kernel/security/lockdown
> $ echo "appraise func=KEXEC_KERNEL_CHECK appraise_type=imasig" > \
>  /sys/kernel/security/ima/policy
> $ kexec -ls unsigned-kernel
> 
> Add a call to verify ima appraisal is set to "enforce" whenever lockdown
> is enabled. This fixes CVE-2022-21505.
> 
> Fixes: 29d3c1c8dfe7 ("kexec: Allow kexec_file() with appropriate IMA policy when locked down")
> Signed-off-by: Eric Snowberg <eric.snowberg@xxxxxxxxxx>
> Acked-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>


Reviewed-by: John Haxby <john.haxby@xxxxxxxxxx>


> ---
> security/integrity/ima/ima_policy.c | 4 ++++
> 1 file changed, 4 insertions(+)
> 
> diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> index 73917413365b..a8802b8da946 100644
> --- a/security/integrity/ima/ima_policy.c
> +++ b/security/integrity/ima/ima_policy.c
> @@ -2247,6 +2247,10 @@ bool ima_appraise_signature(enum kernel_read_file_id id)
> 	if (id >= READING_MAX_ID)
> 		return false;
> 
> +	if (id == READING_KEXEC_IMAGE && !(ima_appraise & IMA_APPRAISE_ENFORCE)
> +	    && security_locked_down(LOCKDOWN_KEXEC))
> +		return false;
> +
> 	func = read_idmap[id] ?: FILE_CHECK;
> 
> 	rcu_read_lock();
> --
> 2.27.0
> 
>

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