Re: [PATCH v9 5/6] efivarfs: force RO when remounting if SetVariable is not supported

[Ilias Apalodimas <ilias.apalodimas@linaro.org>]

Hi Ard,

I found some more issues in the rest of the patches and Kojima-san will
have to respin those.  This is a straight up fix though of an existing
issue.  Any chance you can pick it up separately? If you need any changes
let me know and I can respin it without the rest of the series

Thanks
/Ilias

On Fri, Oct 13, 2023 at 04:45:38PM +0900, Masahisa Kojima wrote:
> From: Ilias Apalodimas <ilias.apalodimas@linaro.org>
>
> If SetVariable at runtime is not supported by the firmware we never assign
> a callback for that function. At the same time mount the efivarfs as
> RO so no one can call that.  However, we never check the permission flags
> when someone remounts the filesystem as RW. As a result this leads to a
> crash looking like this:
>
> $ mount -o remount,rw /sys/firmware/efi/efivars
> $ efi-updatevar -f PK.auth PK
>
> [  303.279166] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
> [  303.280482] Mem abort info:
> [  303.280854]   ESR = 0x0000000086000004
> [  303.281338]   EC = 0x21: IABT (current EL), IL = 32 bits
> [  303.282016]   SET = 0, FnV = 0
> [  303.282414]   EA = 0, S1PTW = 0
> [  303.282821]   FSC = 0x04: level 0 translation fault
> [  303.283771] user pgtable: 4k pages, 48-bit VAs, pgdp=000000004258c000
> [  303.284913] [0000000000000000] pgd=0000000000000000, p4d=0000000000000000
> [  303.286076] Internal error: Oops: 0000000086000004 [#1] PREEMPT SMP
> [  303.286936] Modules linked in: qrtr tpm_tis tpm_tis_core crct10dif_ce arm_smccc_trng rng_core drm fuse ip_tables x_tables ipv6
> [  303.288586] CPU: 1 PID: 755 Comm: efi-updatevar Not tainted 6.3.0-rc1-00108-gc7d0c4695c68 #1
> [  303.289748] Hardware name: Unknown Unknown Product/Unknown Product, BIOS 2023.04-00627-g88336918701d 04/01/2023
> [  303.291150] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> [  303.292123] pc : 0x0
> [  303.292443] lr : efivar_set_variable_locked+0x74/0xec
> [  303.293156] sp : ffff800008673c10
> [  303.293619] x29: ffff800008673c10 x28: ffff0000037e8000 x27: 0000000000000000
> [  303.294592] x26: 0000000000000800 x25: ffff000002467400 x24: 0000000000000027
> [  303.295572] x23: ffffd49ea9832000 x22: ffff0000020c9800 x21: ffff000002467000
> [  303.296566] x20: 0000000000000001 x19: 00000000000007fc x18: 0000000000000000
> [  303.297531] x17: 0000000000000000 x16: 0000000000000000 x15: 0000aaaac807ab54
> [  303.298495] x14: ed37489f673633c0 x13: 71c45c606de13f80 x12: 47464259e219acf4
> [  303.299453] x11: ffff000002af7b01 x10: 0000000000000003 x9 : 0000000000000002
> [  303.300431] x8 : 0000000000000010 x7 : ffffd49ea8973230 x6 : 0000000000a85201
> [  303.301412] x5 : 0000000000000000 x4 : ffff0000020c9800 x3 : 00000000000007fc
> [  303.302370] x2 : 0000000000000027 x1 : ffff000002467400 x0 : ffff000002467000
> [  303.303341] Call trace:
> [  303.303679]  0x0
> [  303.303938]  efivar_entry_set_get_size+0x98/0x16c
> [  303.304585]  efivarfs_file_write+0xd0/0x1a4
> [  303.305148]  vfs_write+0xc4/0x2e4
> [  303.305601]  ksys_write+0x70/0x104
> [  303.306073]  __arm64_sys_write+0x1c/0x28
> [  303.306622]  invoke_syscall+0x48/0x114
> [  303.307156]  el0_svc_common.constprop.0+0x44/0xec
> [  303.307803]  do_el0_svc+0x38/0x98
> [  303.308268]  el0_svc+0x2c/0x84
> [  303.308702]  el0t_64_sync_handler+0xf4/0x120
> [  303.309293]  el0t_64_sync+0x190/0x194
> [  303.309794] Code: ???????? ???????? ???????? ???????? (????????)
> [  303.310612] ---[ end trace 0000000000000000 ]---
>
> Fix this by adding a .reconfigure() function to the fs operations which
> we can use to check the requested flags and deny anything that's not RO
> if the firmware doesn't implement SetVariable at runtime.
>
> Fixes: f88814cc2578 ("efi/efivars: Expose RT service availability via efivars abstraction")
> Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
> ---
>  fs/efivarfs/super.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
>
> diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
> index 0f6e4d223aea..942e748a4e03 100644
> --- a/fs/efivarfs/super.c
> +++ b/fs/efivarfs/super.c
> @@ -15,6 +15,7 @@
>  #include <linux/magic.h>
>  #include <linux/statfs.h>
>  #include <linux/notifier.h>
> +#include <linux/printk.h>
>
>  #include "internal.h"
>
> @@ -300,8 +301,19 @@ static int efivarfs_get_tree(struct fs_context *fc)
>  	return get_tree_single(fc, efivarfs_fill_super);
>  }
>
> +static int efivarfs_reconfigure(struct fs_context *fc)
> +{
> +	if (!efivar_supports_writes() && !(fc->sb_flags & SB_RDONLY)) {
> +		pr_err("Firmware does not support SetVariableRT. Can not remount with rw\n");
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
>  static const struct fs_context_operations efivarfs_context_ops = {
>  	.get_tree	= efivarfs_get_tree,
> +	.reconfigure	= efivarfs_reconfigure,
>  };
>
>  static int efivarfs_init_fs_context(struct fs_context *fc)
> --
> 2.30.2
>