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tower: fix serdes bounds checks #8629

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tower: fix serdes bounds checks #8629

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18 changes: 13 additions & 5 deletions src/choreo/tower/fd_tower_serdes.c
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Original file line number Diff line number Diff line change
Expand Up @@ -16,9 +16,14 @@
} while(0)

static ulong
de_short_u16( ushort * dst, uchar const * src ) {
de_short_u16( ushort * dst,
uchar const * src,
ulong src_sz ) {
if( FD_UNLIKELY( src_sz<1UL ) ) return 0;
if ( FD_LIKELY( !(0x80U & src[0]) ) ) { *dst = (ushort)src[0]; return 1; }
if( FD_UNLIKELY( src_sz<2UL ) ) return 0;
else if( FD_LIKELY( !(0x80U & src[1]) ) ) { *dst = (ushort)((ulong)(src[0]&0x7FUL) + (((ulong)src[1])<<7)); return 2; }
if( FD_UNLIKELY( src_sz<3UL ) ) return 0;
else { *dst = (ushort)((ulong)(src[0]&0x7FUL) + (((ulong)(src[1]&0x7FUL))<<7) + (((ulong)src[2])<<14)); return 3; }
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Copilot AI Mar 2, 2026

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In the 3-byte decode path, src[2] is incorporated without masking/validating and the result is cast to ushort. Inputs where src[2] has bits outside the valid ShortU16 range (e.g. src[2]>3 or src[2]&0x80) will wrap/truncate into an unrelated lockouts_cnt value and can bypass the later max check. Consider enforcing canonical ShortU16 constraints (final byte must terminate and the overall value must fit in ushort).

Suggested change
else { *dst = (ushort)((ulong)(src[0]&0x7FUL) + (((ulong)(src[1]&0x7FUL))<<7) + (((ulong)src[2])<<14)); return 3; }
else {
/* Third byte must terminate (MSB clear) and only use low 2 bits
to keep the value within the 16-bit range and enforce a
canonical ShortU16 encoding. */
if( FD_UNLIKELY( src[2] & 0xFCU ) ) return 0;
ulong val = (ulong)(src[0] & 0x7FUL)
+ (((ulong)(src[1] & 0x7FUL)) << 7)
+ (((ulong)src[2]) << 14);
if( FD_UNLIKELY( val>65535UL ) ) return 0;
*dst = (ushort)val;
return 3;
}

Copilot uses AI. Check for mistakes.
}

Expand Down Expand Up @@ -81,12 +86,15 @@ fd_compact_tower_sync_de( fd_compact_tower_sync_serde_t * serde,
ulong buf_sz ) {
ulong off = 0;
DE( ulong, root );
off += de_short_u16( &serde->lockouts_cnt, buf+off );
ulong n;
n = de_short_u16( &serde->lockouts_cnt, buf+off, buf_sz-off );
if( FD_UNLIKELY( !n ) ) return -1;
off += n;
if( FD_UNLIKELY( serde->lockouts_cnt > FD_TOWER_VOTE_MAX ) ) return -1;
for( ulong i = 0; i < serde->lockouts_cnt; i++ ) {
ulong varint_sz = de_var_int( &serde->lockouts[i].offset, buf+off, buf_sz-off );
if( FD_UNLIKELY( !varint_sz ) ) return -1;
off += varint_sz;
n = de_var_int( &serde->lockouts[i].offset, buf+off, buf_sz-off );
if( FD_UNLIKELY( !n ) ) return -1;
off += n;
DE( uchar, lockouts[i].confirmation_count );
}
DE( fd_hash_t, hash );
Expand Down
5 changes: 2 additions & 3 deletions src/choreo/tower/fuzz_tower_serdes.c
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Original file line number Diff line number Diff line change
Expand Up @@ -13,11 +13,10 @@
int
LLVMFuzzerInitialize( int * argc,
char *** argv ) {
/* Set up shell without signal handlers */
putenv( "FD_LOG_BACKTRACE=0" );
fd_boot( argc, argv );
atexit( fd_halt );
fd_log_level_core_set(3); /* crash on warning log */
fd_log_level_core_set(3);
return 0;
}

Expand All @@ -36,7 +35,7 @@ LLVMFuzzerTestOneInput( uchar const * data,

FD_FUZZ_MUST_BE_COVERED;

uchar buf[1024];
uchar buf[4096];
ulong out_sz = 0;

int ser_err = fd_compact_tower_sync_ser( serde, buf, sizeof(buf), &out_sz );
Expand Down
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