bzero, explicit_bzero - zero a byte string
The bzero() function erases the data in the
n bytes of the memory starting at the location pointed to by
s, by writing zeros (bytes containing '\0') to that area.
The explicit_bzero() function performs the same task as bzero(). It differs from bzero() in that it guarantees that compiler optimizations will not remove the erase operation if the compiler deduces that the operation is "unnecessary".
The explicit_bzero() function addresses a problem that security-conscious applications may run into when using bzero(): if the compiler can deduce that the location to zeroed will never again be touched by a
correct program, then it may remove the bzero() call altogether. This is a problem if the intent of the bzero() call was to erase sensitive data (e.g., passwords) to prevent the possibility that the data was leaked by an incorrect or compromised program. Calls to explicit_bzero() are never optimized away by the compiler.
The explicit_bzero() function does not solve all problems associated with erasing sensitive data:
The explicit_bzero() function does
not guarantee that sensitive data is completely erased from memory. (The same is true of bzero().) For example, there may be copies of the sensitive data in a register and in "scratch" stack areas. The explicit_bzero() function is not aware of these copies, and can't erase them.
In some circumstances, explicit_bzero() can
decrease security. If the compiler determined that the variable containing the sensitive data could be optimized to be stored in a register (because it is small enough to fit in a register, and no operation other than the explicit_bzero() call would need to take the address of the variable), then the explicit_bzero() call will force the data to be copied from the register to a location in RAM that is then immediately erased (while the copy in the register remains unaffected). The problem here is that data in RAM is more likely to be exposed by a bug than data in a register, and thus the explicit_bzero() call creates a brief time window where the sensitive data is more vulnerable than it would otherwise have been if no attempt had been made to erase the data.
Note that declaring the sensitive variable with the volatile qualifier does
not eliminate the above problems. Indeed, it will make them worse, since, for example, it may force a variable that would otherwise have been optimized into a register to instead be maintained in (more vulnerable) RAM for its entire lifetime.
Notwithstanding the above details, for security-conscious applications, using explicit_bzero() is generally preferable to not using it. The developers of explicit_bzero() anticipate that future compilers will recognize calls to explicit_bzero() and take steps to ensure that all copies of the sensitive data are erased, including copies in registers or in "scratch" stack areas.
This page is part of release 5.10 of the Linux
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