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efi_loader: Avoid underflow when calculating remaining var store size

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The efi_var_mem_free() function calculates the available size for a new
EFI variable by subtracting the occupied buffer size and the overhead
for a new variable from the maximum buffer size set in Kconfig. This
is then returned as QueryVariableInfo()'s RemainingVariableStorageSize
output.

This can underflow as the calculation is done in and processed as
unsigned integer types. Check for underflow before doing the subtraction
and return zero if there's no space.

Fixes: f1f990a8c9 ("efi_loader: memory buffer for variables")
Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
This commit is contained in:
Alper Nebi Yasak 2023-07-08 18:23:05 +03:00 committed by Heinrich Schuchardt
parent 0ef6343439
commit d7fe913f23
1 changed files with 4 additions and 0 deletions
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4
lib/efi_loader/efi_var_mem.c
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@ -177,6 +177,10 @@ efi_status_t __efi_runtime efi_var_mem_ins(
u64 __efi_runtime efi_var_mem_free(void)
{
if (efi_var_buf->length + sizeof(struct efi_var_entry) >=
EFI_VAR_BUF_SIZE)
return 0;
return EFI_VAR_BUF_SIZE - efi_var_buf->length -
sizeof(struct efi_var_entry);
}