Re: [PATCH] efivarfs: Rate limit statfs() handler

[Richard Lyu <richard.lyu@suse.com>]

On 2026/08/18 16:11, Ard Biesheuvel wrote:
>Ravi reports that statfs() may be called by unprivileged users on the
>efivarfs mount point, which may result in a flood of calls to the
>QueryVariableInfo() runtime service. These calls are disproportionately
>costly on x86 systems where the variable store is backed by SMM, as each
>SMM entry requires a rendez-vous of all the CPUs.
>
>So rate limit the calls to QueryVariableInfo() at twice per second, and
>return the most recently obtained values for calls that are elided.
>
>Cc: <stable@vger.kernel.org>
>Cc: Anisse Astier <anisse@astier.eu>
>Reported-by: Ravi Bangoria <ravi.bangoria@amd.com>
>Fixes: d86ff3333cb1 ("efivarfs: expose used and total size")
>Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>---
>This supersedes [0], which had some issues, the main one being that a
>cache that requires explicit invalidation may go out of sync due to
>direct calls to SetVariable() made by other drivers.
>
>[0] https://lore.kernel.org/all/20260801144258.15977-2-ardb@kernel.org/
>
> fs/efivarfs/super.c | 30 ++++++++++++++++----
> 1 file changed, 24 insertions(+), 6 deletions(-)
>
>diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
>index 733c19571f1c..8d33f11db2a1 100644
>--- a/fs/efivarfs/super.c
>+++ b/fs/efivarfs/super.c
>@@ -89,12 +89,30 @@ static int efivarfs_statfs(struct dentry *dentry, struct kstatfs *buf)
> 	/* Some UEFI firmware does not implement QueryVariableInfo() */
> 	storage_space = remaining_space = 0;
> 	if (efi_rt_services_supported(EFI_RT_SUPPORTED_QUERY_VARIABLE_INFO)) {
>-		status = efivar_query_variable_info(attr, &storage_space,
>-						    &remaining_space,
>-						    &max_variable_size);
>-		if (status != EFI_SUCCESS && status != EFI_UNSUPPORTED)
>-			pr_warn_ratelimited("query_variable_info() failed: 0x%lx\n",
>-					    status);
>+		static DEFINE_RATELIMIT_STATE(_rs, 2 * HZ, 5);
>+		static u64 storage, remaining;
>+		static DEFINE_SPINLOCK(lock);
>+
>+		if (!__ratelimit(&_rs)) {
>+			ratelimit_set_flags(&_rs, RATELIMIT_MSG_ON_RELEASE);
>+
>+			spin_lock(&lock);
>+			storage_space = storage;
>+			remaining_space = remaining;
>+			spin_unlock(&lock);
>+		} else {
>+			status = efivar_query_variable_info(attr, &storage_space,
>+							    &remaining_space,
>+							    &max_variable_size);
>+			if (status != EFI_SUCCESS && status != EFI_UNSUPPORTED)
>+				pr_warn("query_variable_info() failed: 0x%lx\n",
>+					status);
>+
>+			spin_lock(&lock);
>+			storage = storage_space;
>+			remaining = remaining_space;
>+			spin_unlock(&lock);
>+		}
> 	}
>
> 	/*
>-- 
>2.55.0.699.gb54405d56f-goog
>
>

Reviewed-by: Richard Lyu <richard.lyu@suse.com>