Re: [PATCH v3] ima: Fallback to the builtin hash algorithm

[Mimi Zohar <zohar@linux.vnet.ibm.com>]

On Fri, 2018-03-23 at 14:41 +0100, Petr Vorel wrote:
> IMA requires have it's hash algorithm to be compiled-in due it's early
> use. Default IMA algorithm is protected by Kconfig to be compiled-in.
> 
> ima_hash kernel parameter allows to choose hash algorithm. When
> specified algorithm not available or available as module, IMA
> initialization fails, which leads to kernel panic (mknodat syscall calls
> ima_post_path_mknod()). Therefore as fallback we force IMA to use
> the default builtin Kconfig hash algorithm.
> 
> Fixed crash:
> 
> $ grep CONFIG_CRYPTO_MD4 .config
> CONFIG_CRYPTO_MD4=m
> 
> [    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-4.12.14-2.3-default root=UUID=74ae8202-9ca7-4e39-813b-22287ec52f7a video=1024x768-16 plymouth.ignore-serial-consoles console=ttyS0 console=tty resume=/dev/disk/by-path/pci-0000:00:07.0-part3 splash=silent showopts ima_hash=md4
> ...
> [    1.545190] ima: Can not allocate md4 (reason: -2)
> ...
> [    2.610120] BUG: unable to handle kernel NULL pointer dereference at           (null)
> [    2.611903] IP: ima_match_policy+0x23/0x390
> [    2.612967] PGD 0 P4D 0
> [    2.613080] Oops: 0000 [#1] SMP
> [    2.613080] Modules linked in: autofs4
> [    2.613080] Supported: Yes
> [    2.613080] CPU: 0 PID: 1 Comm: systemd Not tainted 4.12.14-2.3-default #1
> [    2.613080] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.0.0-prebuilt.qemu-project.org 04/01/2014
> [    2.613080] task: ffff88003e2d0040 task.stack: ffffc90000190000
> [    2.613080] RIP: 0010:ima_match_policy+0x23/0x390
> [    2.613080] RSP: 0018:ffffc90000193e88 EFLAGS: 00010296
> [    2.613080] RAX: 0000000000000000 RBX: 000000000000000c RCX: 0000000000000004
> [    2.613080] RDX: 0000000000000010 RSI: 0000000000000001 RDI: ffff880037071728
> [    2.613080] RBP: 0000000000008000 R08: 0000000000000000 R09: 0000000000000000
> [    2.613080] R10: 0000000000000008 R11: 61c8864680b583eb R12: 00005580ff10086f
> [    2.613080] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000008000
> [    2.613080] FS:  00007f5c1da08940(0000) GS:ffff88003fc00000(0000) knlGS:0000000000000000
> [    2.613080] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [    2.613080] CR2: 0000000000000000 CR3: 0000000037002000 CR4: 00000000003406f0
> [    2.613080] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> [    2.613080] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> [    2.613080] Call Trace:
> [    2.613080]  ? shmem_mknod+0xbf/0xd0
> [    2.613080]  ima_post_path_mknod+0x1c/0x40
> [    2.613080]  SyS_mknod+0x210/0x220
> [    2.613080]  entry_SYSCALL_64_fastpath+0x1a/0xa5
> [    2.613080] RIP: 0033:0x7f5c1bfde570
> [    2.613080] RSP: 002b:00007ffde1c90dc8 EFLAGS: 00000246 ORIG_RAX: 0000000000000085
> [    2.613080] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f5c1bfde570
> [    2.613080] RDX: 0000000000000000 RSI: 0000000000008000 RDI: 00005580ff10086f
> [    2.613080] RBP: 00007ffde1c91040 R08: 00005580ff10086f R09: 0000000000000000
> [    2.613080] R10: 0000000000104000 R11: 0000000000000246 R12: 00005580ffb99660
> [    2.613080] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000002
> [    2.613080] Code: 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 41 57 41 56 44 8d 14 09 41 55 41 54 55 53 44 89 d3 09 cb 48 83 ec 38 48 8b 05 c5 03 29 01 <4c> 8b 20 4c 39 e0 0f 84 d7 01 00 00 4c 89 44 24 08 89 54 24 20
> [    2.613080] RIP: ima_match_policy+0x23/0x390 RSP: ffffc90000193e88
> [    2.613080] CR2: 0000000000000000
> [    2.613080] ---[ end trace 9a9f0a8a73079f6a ]---
> [    2.673052] Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000009
> [    2.673052]
> [    2.675337] Kernel Offset: disabled
> [    2.676405] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x00000009
> 
> Signed-off-by: Petr Vorel <pvorel@suse.cz>

Thanks!

Mimi

> ---
> Changes v2->v3:
> * Return back to v1 variant + log in ima_init_crypto() and in
> init_ima().
> 
> I choose not to detect and mention in ima_init_crypto() whether
> allocated hash is default or not (I don't consider it important enough
> if everything work well and don't want to say "default" when some of
> working algorithms actually aren't default, e.g. md5).
> 
> But I decide to put this info in init_ima() in case we load default
> algorithm as fallback.
> 
> $ grep -e ^CONFIG_IMA_DEFAULT_HASH -e CONFIG_CRYPTO_MD[45] .config
> CONFIG_IMA_DEFAULT_HASH_SHA256=y
> CONFIG_IMA_DEFAULT_HASH="sha256"
> CONFIG_CRYPTO_MD4=m
> CONFIG_CRYPTO_MD5=y
> 
> * default (none) or ima_hash=sha256
> [    2.162025] ima: Allocated hash algorithm: sha256
> 
> * ima_hash=md4
> [    2.136885] ima: Can not allocate md4 (reason: -2)
> [    2.138364] ima: Allocating md4 failed, going to use default hash algorithm sha256
> [    2.141472] ima: Allocated hash algorithm: sha256
> 
> * ima_hash=md5
> [    2.128360] ima: Allocated hash algorithm: md5
> ---
>  security/integrity/ima/ima_crypto.c |  2 ++
>  security/integrity/ima/ima_main.c   | 13 +++++++++++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/security/integrity/ima/ima_crypto.c b/security/integrity/ima/ima_crypto.c
> index 205bc69361ea..4e085a17124f 100644
> --- a/security/integrity/ima/ima_crypto.c
> +++ b/security/integrity/ima/ima_crypto.c
> @@ -73,6 +73,8 @@ int __init ima_init_crypto(void)
>  		       hash_algo_name[ima_hash_algo], rc);
>  		return rc;
>  	}
> +	pr_info("Allocated hash algorithm: %s\n",
> +		hash_algo_name[ima_hash_algo]);
>  	return 0;
>  }
> 
> diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> index 5d122daf5c8a..74d0bd7e76d7 100644
> --- a/security/integrity/ima/ima_main.c
> +++ b/security/integrity/ima/ima_main.c
> @@ -16,6 +16,9 @@
>   *	implements the IMA hooks: ima_bprm_check, ima_file_mmap,
>   *	and ima_file_check.
>   */
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
>  #include <linux/module.h>
>  #include <linux/file.h>
>  #include <linux/binfmts.h>
> @@ -504,6 +507,16 @@ static int __init init_ima(void)
>  	ima_init_template_list();
>  	hash_setup(CONFIG_IMA_DEFAULT_HASH);
>  	error = ima_init();
> +
> +	if (error && strcmp(hash_algo_name[ima_hash_algo],
> +			    CONFIG_IMA_DEFAULT_HASH) != 0) {
> +		pr_info("Allocating %s failed, going to use default hash algorithm %s\n",
> +			hash_algo_name[ima_hash_algo], CONFIG_IMA_DEFAULT_HASH);
> +		hash_setup_done = 0;
> +		hash_setup(CONFIG_IMA_DEFAULT_HASH);
> +		error = ima_init();
> +	}
> +
>  	if (!error) {
>  		ima_initialized = 1;
>  		ima_update_policy_flag();