efi: Fix a race and a buffer overflow while reading efivars via sysfs

There is a race and a buffer overflow corrupting a kernel memory while
reading an EFI variable with a size more than 1024 bytes via the older
sysfs method. This happens because accessing struct efi_variable in
efivar_{attr,size,data}_read() and friends is not protected from
a concurrent access leading to a kernel memory corruption and, at best,
to a crash. The race scenario is the following:

CPU0:                                CPU1:
efivar_attr_read()
  var->DataSize = 1024;
  efivar_entry_get(... &var->DataSize)
    down_interruptible(&efivars_lock)
                                     efivar_attr_read() // same EFI var
                                       var->DataSize = 1024;
                                       efivar_entry_get(... &var->DataSize)
                                         down_interruptible(&efivars_lock)
    virt_efi_get_variable()
    // returns EFI_BUFFER_TOO_SMALL but
    // var->DataSize is set to a real
    // var size more than 1024 bytes
    up(&efivars_lock)
                                         virt_efi_get_variable()
                                         // called with var->DataSize set
                                         // to a real var size, returns
                                         // successfully and overwrites
                                         // a 1024-bytes kernel buffer
                                         up(&efivars_lock)

This can be reproduced by concurrent reading of an EFI variable which size
is more than 1024 bytes:

  ts# for cpu in $(seq 0 $(nproc --ignore=1)); do ( taskset -c $cpu \
  cat /sys/firmware/efi/vars/KEKDefault*/size & ) ; done

Fix this by using a local variable for a var's data buffer size so it
does not get overwritten.

Fixes: e14ab23 ("efivars: efivar_entry API")
Reported-by: Bob Sanders <bob.sanders@hpe.com> and the LTP testsuite
Signed-off-by: Vladis Dronov <vdronov@redhat.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20200305084041.24053-2-vdronov@redhat.com
Link: https://lore.kernel.org/r/20200308080859.21568-24-ardb@kernel.org

drivers/firmware/efi/efivars.c

@@ -83,13 +83,16 @@ static ssize_t
 efivar_attr_read(struct efivar_entry *entry, char *buf)
 {
 	struct efi_variable *var = &entry->var;
+	unsigned long size = sizeof(var->Data);
 	char *str = buf;
+	int ret;
 
 	if (!entry || !buf)
 		return -EINVAL;
 
-	var->DataSize = 1024;
-	if (efivar_entry_get(entry, &var->Attributes, &var->DataSize, var->Data))
+	ret = efivar_entry_get(entry, &var->Attributes, &size, var->Data);
+	var->DataSize = size;
+	if (ret)
 		return -EIO;
 
 	if (var->Attributes & EFI_VARIABLE_NON_VOLATILE)
@@ -116,13 +119,16 @@ static ssize_t
 efivar_size_read(struct efivar_entry *entry, char *buf)
 {
 	struct efi_variable *var = &entry->var;
+	unsigned long size = sizeof(var->Data);
 	char *str = buf;
+	int ret;
 
 	if (!entry || !buf)
 		return -EINVAL;
 
-	var->DataSize = 1024;
-	if (efivar_entry_get(entry, &var->Attributes, &var->DataSize, var->Data))
+	ret = efivar_entry_get(entry, &var->Attributes, &size, var->Data);
+	var->DataSize = size;
+	if (ret)
 		return -EIO;
 
 	str += sprintf(str, "0x%lx\n", var->DataSize);
@@ -133,12 +139,15 @@ static ssize_t
 efivar_data_read(struct efivar_entry *entry, char *buf)
 {
 	struct efi_variable *var = &entry->var;
+	unsigned long size = sizeof(var->Data);
+	int ret;
 
 	if (!entry || !buf)
 		return -EINVAL;
 
-	var->DataSize = 1024;
-	if (efivar_entry_get(entry, &var->Attributes, &var->DataSize, var->Data))
+	ret = efivar_entry_get(entry, &var->Attributes, &size, var->Data);
+	var->DataSize = size;
+	if (ret)
 		return -EIO;
 
 	memcpy(buf, var->Data, var->DataSize);
@@ -250,14 +259,16 @@ efivar_show_raw(struct efivar_entry *entry, char *buf)
 {
 	struct efi_variable *var = &entry->var;
 	struct compat_efi_variable *compat;
+	unsigned long datasize = sizeof(var->Data);
 	size_t size;
+	int ret;
 
 	if (!entry || !buf)
 		return 0;
 
-	var->DataSize = 1024;
-	if (efivar_entry_get(entry, &entry->var.Attributes,
-			     &entry->var.DataSize, entry->var.Data))
+	ret = efivar_entry_get(entry, &var->Attributes, &datasize, var->Data);
+	var->DataSize = datasize;
+	if (ret)
 		return -EIO;
 
 	if (in_compat_syscall()) {