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fatfs: optional AES-256-XTS full-disk encryption hook #105

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fatfs: optional AES-256-XTS full-disk encryption hook #105

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47 changes: 47 additions & 0 deletions os/various/fatfs_bindings/fatfs_diskio.c
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Original file line number Diff line number Diff line change
Expand Up @@ -30,6 +30,26 @@ extern SDCDriver FATFS_HAL_DEVICE;
extern RTCDriver RTCD1;
#endif

/*
Optional transparent full-disk AES-256-XTS encryption of the SD card,
implemented in ArduPilot's AP_DiskCrypto library. Turned on with
"define AP_FATFS_CRYPTO_ENABLED 1" in a board hwdef, which reaches here via
ffconf.h -> hwdef.h. It is intentionally excluded from the bootloader, which
must access the card without linking the encryption library.
*/
#if defined(AP_FATFS_CRYPTO_ENABLED) && AP_FATFS_CRYPTO_ENABLED && !defined(HAL_BOOTLOADER_BUILD)
#include <stdbool.h>
#define FATFS_CRYPTO_ACTIVE 1
/* SD/MMC sectors are always 512 bytes (FF_MIN_SS == FF_MAX_SS) */
#define FATFS_CRYPTO_SECTOR_SIZE 512U
/* sectors encrypted per blkWrite chunk; bounds the static write scratch buffer */
#define FATFS_CRYPTO_WRITE_CHUNK 8U
bool ap_diskcrypto_encrypt(const uint8_t *in, uint8_t *out, uint32_t lba, uint32_t nsectors);
bool ap_diskcrypto_decrypt(uint8_t *buf, uint32_t lba, uint32_t nsectors);
#else
#define FATFS_CRYPTO_ACTIVE 0
#endif

/*-----------------------------------------------------------------------*/
/* Correspondence between physical drive number and physical drive. */

Expand Down Expand Up @@ -106,6 +126,11 @@ DRESULT disk_read (
if (blkGetDriverState(&FATFS_HAL_DEVICE) != BLK_READY)
return RES_NOTRDY;
FATFS_RETRY(blkRead(&FATFS_HAL_DEVICE, sector, buff, count));
#if FATFS_CRYPTO_ACTIVE
/* decrypt the sectors in place after they are read from the card */
if (!ap_diskcrypto_decrypt(buff, sector, count))
return RES_ERROR;
#endif
return RES_OK;
}
return RES_PARERR;
Expand All @@ -128,7 +153,29 @@ DRESULT disk_write (
case 0:
if (blkGetDriverState(&FATFS_HAL_DEVICE) != BLK_READY)
return RES_NOTRDY;
#if FATFS_CRYPTO_ACTIVE
{
/*
Encrypt into a bounded scratch buffer before writing. The caller's
buffer is const and may be reused by FatFS, so it cannot be encrypted
in place. Large writes are processed in FATFS_CRYPTO_WRITE_CHUNK
sector chunks to keep the scratch buffer small.
*/
static uint8_t crypto_buf[FATFS_CRYPTO_WRITE_CHUNK * FATFS_CRYPTO_SECTOR_SIZE];
UINT done = 0;
while (done < count) {
UINT n = count - done;
if (n > FATFS_CRYPTO_WRITE_CHUNK)
n = FATFS_CRYPTO_WRITE_CHUNK;
if (!ap_diskcrypto_encrypt(buff + (done * FATFS_CRYPTO_SECTOR_SIZE), crypto_buf, sector + done, n))
return RES_ERROR;
FATFS_RETRY(blkWrite(&FATFS_HAL_DEVICE, sector + done, crypto_buf, n));
done += n;
}
}
#else
FATFS_RETRY(blkWrite(&FATFS_HAL_DEVICE, sector, buff, count));
#endif
return RES_OK;
}
return RES_PARERR;
Expand Down